Electrochemical hybrid catalysts containing nickel-iron diatomic metals and carbon dioxide conversion systems using them

Abstract

To provide: a transition metal based electrochemical electrode catalyst available for carbon dioxide reduction that improves price competitiveness with better activity than noble metal catalysts such as Ag / C; a production method thereof; an electrode for carbon dioxide reduction reaction containing the catalyst; and a carbon dioxide reduction system including the catalyst.SOLUTION: An electrode catalyst comprises: a support comprising a nitrogen-doped carbon nanostructure; and Ni and Fe in the form of single atoms each supported on the support, where the Ni is bonded to four surrounding nitrogen atoms while the Fe is bonded to five surrounding nitrogen atoms, and where the Ni and Fe at the respective sites are adjacent to each other and are indirectly linked to each other via nitrogen, thereby forming an active site. The electrode catalyst exhibits high carbon monoxide selectivity and current density under a low overpotential condition during reduction of carbon dioxide to carbon monoxide.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an electrochemical hybrid catalyst containing nickel (Ni)-iron (Fe) diatomic metals and a carbon dioxide conversion system using the same. More specifically, the present disclosure relates to an electrochemical hybrid catalyst that exhibits high carbon monoxide selectivity and current density under low overpotential conditions during the reduction reaction of carbon dioxide to carbon monoxide by forming catalytic sites or active sites in a nitrogen (N)-doped carbon nanostructure ((NC) nanostructure) composite in which two monatomic metals, nickel and iron, each bonded (or coordinated) to nitrogen, are adjacent to each other and indirectly linked via the nitrogen (N). The catalyst and carbon dioxide conversion system using the same. [Background technology]

[0002] Since the Industrial Revolution, the amount of carbon dioxide (CO2) emitted has been increasing exponentially along with the massive consumption of fossil fuels and the global population growth. Carbon dioxide is emitted into the atmosphere through human industrial and economic activities, and by adding to the existing carbon cycle, it increases the greenhouse effect. This increases the temperature on the earth's surface and in the lower atmosphere, triggering global warming and climate change, and is emerging as a fatal factor threatening the ecosystem.

[0003] As a result, countries around the world are announcing greenhouse gas reduction plans, and the burden of reducing carbon dioxide emissions is increasing across the industrial sector. While existing research has focused primarily on carbon dioxide capture and storage, recently, intensive research has been conducted on technologies to turn generated carbon dioxide into resources, i.e., technologies to convert carbon dioxide into useful substances such as high-value-added fuels or platform compounds, as a way to realistically contribute to the 2030 greenhouse gas emission reduction target of 37% compared to the business as usual (BAU) scenario, as specified in the 2015 Paris Climate Change Agreement.

[0004] Carbon dioxide conversion technologies include electrochemical conversion, photochemical conversion, catalytic chemical conversion, and biological conversion. Electrochemical conversion, in particular, (i) can be operated under ambient temperature and pressure conditions, (ii) has a simple and modular system, (iii) can increase the net carbon dioxide reduction rate when using renewable energy, and (iv) can be used as a renewable energy storage technology by converting carbon dioxide into high-value-added chemicals. In particular, the carbon dioxide reduction reaction (CO2RR), which reacts thermodynamically stable carbon dioxide with water, can produce various organic compounds, minimizing pollutant emissions and being environmentally friendly. Thus, the high-value-added chemicals and fuels produced by the CO2 reduction reaction are promising solutions to the carbon cycle, energy crisis, and environmental pollution on a commercial scale.

[0005] However, previously studied electrochemical CO2 reduction processes have been limited by low reaction rates, variable product distribution (low selectivity), and the occurrence of competing reactions such as hydrogen evolution reactions (HER). To overcome these technological barriers, electrochemical catalysts with higher catalytic activity and efficiency for CO2 reduction are required. Existing research has proposed various electrochemical CO2 conversion catalysts, including metal-free catalysts, metal-based catalysts, and metal-organic framework (MOF)-based catalysts, which offer high activity and selectivity. In particular, the use of noble metal catalysts such as gold (Au) and silver (Ag) among metal-based catalysts has been reported to yield excellent results in terms of current density and faradaic efficiency (FE) for carbon monoxide (CO).

[0006] However, precious metal catalysts are disadvantageous from a commercial perspective due to their use of expensive precious metals. Recently, interest has been growing in catalysts in which metals are dispersed (or supported) at single-atom sizes, i.e., single-atom catalysts (SACs). Single-atom catalysts can maximize carbon dioxide conversion activity by reducing the size of metal active sites to the sub-nanoatomic level and provide a solution for replacing expensive precious metals with inexpensive transition metals. In particular, their extremely simplified and idealized structure makes them suitable for related research, simulating theoretical results obtained through computational science (e.g., J. Phys. Chem. C, 2013, 117, 9187-9195; Angew. Chem. Int. Ed., 2015, 54, 10758-10762). However, metal catalysts with single-atom sizes tend to aggregate and form particles, making it difficult to commercially synthesize stable single-atom catalysts. The costly and complicated synthesis process poses a barrier to their application in various fields. In particular, the structural simplicity of single-atom active sites limits the improvement of catalytic efficiency in the carbon dioxide reduction process, which involves multi-step proton-electron transfer.

[0007] Therefore, carbon-based metal-nitrogen catalysts ((MN) catalysts) that provide single-atom transition metal active sites have been developed. These catalysts exhibit low cost, tunable structures, high activity, and stability. Furthermore, their favorable coordination environment can promote the formation of active sites and catalytic mechanisms.

[0008] Currently, nickel (Ni) single-atom catalysts are known to be effective alternatives to noble metal catalysts because they exhibit high activity and selectivity for carbon monoxide in carbon dioxide conversion reactions. However, they still require a large amount of energy (high overpotential) to achieve high faradaic efficiency.

[0009] Therefore, there is a need for techniques to adjust the electronic structure of single-atom catalysts to improve their carbon dioxide conversion performance. Summary of the Invention [Problem to be solved by the invention]

[0010] In one embodiment of the present disclosure, there is provided a transition metal single-atom catalyst and an electrochemical electrode including the same, which can provide superior carbon dioxide conversion activity and improve price competitiveness compared to existing commercial noble metal catalysts (e.g., Ag / C).

[0011] Another embodiment of the present disclosure provides a carbon dioxide conversion system that employs a catalyst that has higher carbon dioxide conversion activity and higher selectivity for carbon monoxide than conventional technologies, and that can effectively remove carbon dioxide from exhaust gases emitted from power plants, steel mills, and the like. [Means for solving the problem]

[0012] According to the first invention, (i) a support comprising nitrogen-doped carbon nanostructures ((NC) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; Including, where nickel (Ni) is linked to each of the four surrounding nitrogen atoms, while iron (Fe) is linked to each of the five surrounding nitrogen atoms, forming Ni-N4 sites and Fe-N5 sites; and A catalyst for carbon dioxide conversion is provided in which Ni at the Ni-N4 site and Fe at the Fe-N5 site are adjacent to each other and indirectly connected via nitrogen.

[0013] According to the second invention, a) forming an iron (Fe)-containing polymer / carbon composite by attaching a nitrogen-containing monomer to carbon and polymerizing in situ using an iron-based oxidizing agent; b) depositing a nickel precursor onto the iron (Fe)-containing polymer / carbon composite to form a NiFe-polymer / carbon composite; c) pyrolyzing the NiFe-polymer / carbon composite to deposit nickel (Ni) and iron (Fe) in monoatomic form on a support comprising nitrogen-doped carbon (NC) nanostructures; Including, Here, nickel (Ni) is linked to each of the four surrounding nitrogen atoms, while iron (Fe) is linked to each of the five surrounding nitrogen atoms to form Ni-N4 sites and Fe-N5 sites, and the Ni in the Ni-N4 sites and the Fe in the Fe-N5 sites are adjacent to each other via nitrogen and indirectly linked to each other via nitrogen.

[0014] According to the third invention, Electrode substrate; and a bimetallic catalyst loaded on the electrode substrate; An electrode for a carbon dioxide reduction reaction comprising: The diatomic metal catalyst is (i) a support comprising nitrogen-doped carbon nanostructures ((NC) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; an electrochemical catalyst comprising In the bimetallic catalyst, nickel (Ni) is linked to each of the four surrounding nitrogen atoms, while iron (Fe) is linked to each of the five surrounding nitrogen atoms, forming Ni-N4 sites and Fe-N5 sites; and An electrode is provided in which Ni at the Ni-N4 site and Fe at the Fe-N5 site are adjacent to each other via nitrogen and indirectly connected to each other.

[0015] According to an exemplary embodiment, the loading of the bimetallic catalyst in the electrode is 0.2 to 5 mg / cm. 2 may be in the range of

[0016] According to the fourth invention, an anode and a cathode as electrochemical electrodes electrically connected to an external power source; and an aqueous electrolyte filled between the anode and the cathode; A carbon dioxide reduction system comprising: When a voltage is applied from the external power supply, oxygen is generated from the anode side, and carbon monoxide is generated from the cathode side. the cathode comprises a bimetallic catalyst loaded on an electrode substrate; wherein the diatomic metal catalyst is (i) a support comprising nitrogen-doped carbon nanostructures ((NC) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; an electrochemical catalyst comprising In the bimetallic catalyst, nickel (Ni) is linked to each of the four surrounding nitrogen atoms, while iron (Fe) is linked to each of the five surrounding nitrogen atoms, forming Ni-N4 sites and Fe-N5 sites; and A system is provided in which Ni at the Ni-N4 site and Fe at the Fe-N5 site are adjacent to each other and indirectly linked via nitrogen. [Effects of the Invention]

[0017] In the nickel (Ni)-iron (Fe) bimetallic catalyst according to an embodiment of the present disclosure, two monoatomic transition metals (i.e., nickel (Ni) and iron (Fe)) are bonded (or coordinated) to nitrogen atoms in a nitrogen-doped carbon nanostructure and supported in an indirectly linked (or bonded) state via the nitrogen atom. As a result, the nickel (Ni)-iron (Fe) bimetallic catalyst can achieve better carbon dioxide conversion performance (low overpotential, high current density, and high carbon monoxide selectivity) than monometallic monoatomic catalysts or biatomic catalysts in which nickel (Ni) and iron (Fe) are directly bonded in a monoatomic state. Furthermore, the nickel (Ni)-iron (Fe) bimetallic catalyst can provide improved electrochemical catalytic activity in the carbon dioxide reduction reaction compared to commercial noble metal-based catalysts, even while using inexpensive transition metals or base metals, and is therefore economically advantageous. The nickel (Ni)-iron (Fe) bimetallic catalyst according to this embodiment can be produced by a simple procedure such as ion adsorption, and can be applied to the electrode (i.e., cathode) of an electrochemical system for carbon dioxide reduction reaction, where it can reduce carbon dioxide while effectively producing highly-added compounds (e.g., carbon monoxide, a component of synthesis gas). Therefore, it is expected to be widely applied in the future. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram illustrating a procedure for preparing a nickel (Ni)-iron (Fe) bimetallic catalyst (NiFe-DAC) according to an exemplary embodiment. [Figure 2A] 1 is a HAADF-STEM / element mapping photograph of a nickel (Ni)-iron (Fe) bimetallic catalyst (NiFe-DAC) according to an example. [Figure 2B] 1 is a HAADF-STEM / element mapping photograph of a nickel (Ni)-iron (Fe) bimetallic catalyst (NiFe-DAC) according to an example. [Figure 2C] 1 is a HAADF-STEM / element mapping photograph of a nickel (Ni)-iron (Fe) bimetallic catalyst (NiFe-DAC) according to an example. [Figure 3] 1 shows X-ray diffraction (XRD) patterns of a nickel (Ni)-iron (Fe) biatomic metal catalyst (NiFe-DAC) according to an example and a single Fe atom catalyst (Fe-SAC) according to a comparative example. [Figure 4] 1 is a graph showing the results of XPS (X-ray Photoelectron Spectroscopy) analysis of a nickel (Ni)-iron (Fe) diatomic metal catalyst (NiFe-DAC) according to an example and a single Fe atom catalyst (Fe-SAC) according to a comparative example. [Figure 5A] 1 is a graph showing the results of EXAFS (Extended X-ray absorption fine structure) analysis of a nickel (Ni)-iron (Fe) diatomic metal catalyst (NiFe-DAC) according to an example and a single Fe atom catalyst (Fe-SAC) according to a comparative example. [Figure 5B] 1 is a graph showing the results of an EXAFS (Extended X-ray absorption fine structure) analysis of a nickel (Ni)-iron (Fe) diatomic metal catalyst (direct bond; NiFe-DAC(Direct)) according to a comparative example. [Figure 6] 1 is a graph showing the results of BET analysis of a catalyst (NiFe-DAC) according to an example and a catalyst (Fe-SAC) according to a comparative example. [Figure 7] 1 is a graph showing the faradaic efficiency (FE) for carbon monoxide production for a nickel (Ni)-iron (Fe) diatomic metal catalyst (NiFe-DAC) according to an example, and a single Fe catalyst (Fe-SAC) according to a comparative example, a commercial Ag / C catalyst, and a nickel (Ni)-iron (Fe) diatomic metal catalyst (direct bond; NiFe-DAC(Direct)). DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention can be achieved by the following description. The following description should be understood as describing a preferred embodiment of the present invention, and the present invention is not necessarily limited thereby. In addition, the accompanying drawings are for the purpose of aiding understanding, and the present invention is not limited thereto. Details of individual configurations can be properly understood by the specific meaning of the related description below.

[0020] As used herein, the term "catalyst" refers to a component that increases the rate of an electrochemical reaction and participates in the reaction without being consumed by the reaction itself. In a narrower sense, the term may refer to a component that promotes the reaction mechanism of donating or accepting electrons in the carbon dioxide reduction reaction to produce carbon monoxide and / or the oxygen production reaction.

[0021] "Electrochemical catalyst" means a catalyst that participates in an electrochemical reaction, typically functioning at an electrode surface or may be the electrode surface itself.

[0022] The term "electrochemical carbon dioxide conversion reaction" refers generally to a reaction in which carbon dioxide is reduced to useful carbon compounds by inputting electrical energy to generate a potential difference between two electrodes (a cathode and an anode) and inducing the transfer of electrons, more specifically, a reaction in which carbon dioxide is reduced to carbon monoxide.

[0023] "Nanostructure" refers to any nanoscale object having a feature or texture with a dimension or size on the nanoscale (e.g., about 0.1 to 1000 nm, preferably 1 to 100 nm), such as a nanofiller, nanorod, nano-wall, nanowire, nanoweb, etc.

[0024] "Single atom form" is understood to mean a form in which the active metal attached or fixed to the support is present in isolated or individualized atomic form.

[0025] "Indirect" means that two atoms or ions are not directly connected to each other, but are connected through another atom or group. Conversely, "direct" means that two atoms are directly connected to each other (e.g., metal-metal bonding).

[0026] As used herein, the term "electrode" refers to a cathode in a carbon dioxide reduction reaction (CORR) system where a reduction reaction of carbon dioxide occurs and an anode where an oxidation reaction occurs, and is typically a conductive structure that is externally separated from the external power source and is electrically connected to an external power source.

[0027] "Catalytic electrode" means a current collector loaded with a catalyst absorbed on or separately electrically connected to a current collector, where the catalyst is understood to become associated with the current collector by changing its oxidation state and / or undergoing dynamic equilibrium with the aqueous medium when exposed to an aqueous medium (e.g., an aqueous electrolyte solution).

[0028] "Electrochemical electrode" refers to an electrode structure in which a catalytic component that promotes the carbon dioxide reduction reaction is mixed into or attached to the substrate of the electrode (cathode and / or anode).

[0029] "Overpotential" means a voltage other than the thermodynamic reduction or oxidation voltage required to obtain a desired catalytic activity. As used herein, overpotential refers to the voltage that must be applied to an electrode to cause the carbon dioxide reduction reaction to occur minus the thermodynamic voltage required for that reaction.

[0030] "Pyrolysis" refers to a chemical process in which certain substances are converted into other substances by heat, with or without gases, typically the pyrolysis of organic substances to form gaseous and / or liquid products, which may produce a solid residue with increased carbon content. Pyrolysis occurs in an inert gas (e.g., nitrogen, argon, etc.) or reactive gas (e.g., ammonia, hydrogen, etc.) atmosphere.

[0031] Terms such as "left side" or "right side" may be understood to describe the relative positional relationship between components or members, and the terms "located on the left side" or "located on the right side" are understood to express the relative positional relationship both in a state of contact with a specific object as well as in a state of not contacting the specific object.

[0032] In this specification, when a numerical range is specified as a lower limit and / or an upper limit, it is understood that any other combination of numerical ranges within that numerical range is also disclosed. For example, when described as "1 to 5," it may include not only 1, 2, 3, 4, and 5, but also any other combination of numerical ranges therebetween.

[0033] In this specification, when any component or member is described as being "connected" to another component or member, unless otherwise specified, it is understood that this includes not only when it is directly connected to the other component or member, but also when it is connected via another component or member.

[0034] Similarly, the term "contact" is understood to include not only direct contact but also contact via another component or member.

[0035] When a certain component is referred to as "comprising," this means that other components may also be included, unless otherwise specified.

[0036] Single-atom catalysts for carbon dioxide conversion An electrochemical catalyst according to an embodiment of the present disclosure may be used as a cathode catalyst in a carbon dioxide reduction system in which an anode (oxidizing electrode) and a cathode (reducing electrode) are electrically connected to an external power source and an electrolyte (specifically, an aqueous electrolyte) is filled between the anode and cathode. This electrochemical carbon dioxide conversion reaction is a process in which carbon dioxide is electrolyzed together with an aqueous medium to obtain a desired product. Using electrical energy supplied from an external power source, a carbon dioxide reduction reaction occurs at the cathode (reducing electrode) as shown in Reaction Scheme 1 below, and an oxygen production reaction occurs via an oxidation reaction at the anode (oxidizing electrode) as shown in Reaction Scheme 2 below.

[0037] [Reaction Scheme 1] CO2+H2O+2e - → CO+2OH -

[0038] [Reaction Scheme 2] 2OH - → 1 / 2O2+H2O+2e -

[0039] In particular, the detailed mechanism of the carbon dioxide reduction reaction at the cathode can be expressed as shown in Reaction Scheme 3 below.

[0040] [ka]

[0041] According to this embodiment, an electrochemical catalyst for promoting the carbon dioxide reduction reaction (CORR) occurring at the cathode is provided, in which two metals, i.e., nickel (Ni) and iron (Fe), are supported on a nitrogen-doped carbon nanostructure in a monoatomic form. The catalyst is a hybrid catalyst in which nickel and iron coexist in a monoatomic form, adjacent to each other in an atomic state (monoatomic form). As a result, both carbon dioxide adsorption and activation and efficient carbon monoxide desorption are effectively achieved in the carbon dioxide conversion mechanism according to Reaction 3.

[0042] It should also be noted that because nickel and iron are not directly linked to each other but interact indirectly linked via nitrogen, such catalysts can achieve high carbon dioxide conversion and carbon monoxide selectivity even under low overpotential conditions compared to monometallic monoatomic catalysts (e.g., Fe-monatomic catalysts and Ni-monatomic catalysts) and diatomic catalysts (DAC) in which nickel (Ni) and iron (Fe) are directly linked to each other.

[0043] In one embodiment, the bimetallic catalyst comprises nickel (Ni) and iron (Fe) each in monoatomic form dispersed or supported on a support comprising nitrogen-doped carbon nanostructures.

[0044] According to an exemplary embodiment, the bimetallic catalyst may exhibit porosity. Illustratively, the bimetallic catalyst may exhibit micropore (preferably, 2 nm or less) porosity. In this case, the specific surface area (BET) of the catalyst may be, for example, about 100 to 300 m. 2 / g, preferably about 120 to 250 m 2 / g, more preferably about 130 to 180 m 2 / g, and the pore volume is about 0.1 to 0.5 cm 3 / g, preferably about 0.15 to 0.4 cm 3 / g, more preferably about 0.2 to 0.3 cm 3 / g. The numerical ranges set forth above are to be understood as exemplary only.

[0045] According to an exemplary embodiment, the nitrogen content in a nitrogen-doped carbon nanostructure that can be used as a support may be, for example, in the range of about 1 to 12 atomic %, preferably about 2 to 10 atomic %, and more preferably about 4 to 8 atomic %. In this regard, the amount of nitrogen doping is a factor that affects the degree of bonding with metal atoms, and it is preferable to appropriately adjust it within the above-mentioned range. However, the present disclosure is not limited thereto.

[0046] According to this embodiment, in the carbon dioxide conversion catalyst, two transition metals, i.e., nickel (Ni) and iron (Fe), are supported in a single-atom form or an isolated form on an atomic scale so as to form active sites during the carbon dioxide conversion reaction and to form an electronic structure of the active sites suitable for the carbon dioxide conversion reaction through interaction.

[0047] Although the present disclosure is not bound by any particular theory, the reason for adopting a combination or pair of nickel and iron as diatomic metals (transition metals) in a carbon dioxide conversion catalyst may be explained as follows: nickel has properties that are favorable for carbon monoxide desorption, and iron is favorable for the activation step (formation of the COOH reaction intermediate), which is the initial step in carbon dioxide conversion. Thus, by configuring the active metals in the catalyst as a combination or pair of nickel and iron, it is possible to change the electronic structure by using effective electron transfer between nickel and iron. As a result, by adjusting the activation energy of the carbon dioxide conversion step, it is possible to ultimately maximize catalytic performance, which is considered to be an advantage over other diatomic transition metal combinations.

[0048] According to exemplary embodiments, the biatomic metallic catalysts according to the present invention may optionally further comprise other metals or transition metals in addition to the combination of nickel and iron for further modification or modification, in which case the additional metals may also be introduced in monoatomic form.

[0049] According to this embodiment, nickel and iron may each be supported in a form bonded (specifically, coordinated) with nitrogen atoms in the nitrogen-doped carbon nanostructure. Specifically, nickel (Ni) is linked to each of the four surrounding nitrogen atoms to form Ni-N4 sites, and iron (Fe) is linked to each of the five surrounding nitrogen atoms to form Fe-N5 sites. These Ni-N4 sites and Fe-N5 sites can act as active sites for the carbon dioxide reduction reaction.

[0050] It should be noted that, according to one embodiment, the nickel and iron in the catalyst are adjacent to each other but indirectly connected via nitrogen, unlike the conventionally known Ni-Fe diatomic catalysts in which the Ni and Fe are directly connected. In this case, the nitrogen interposed between the nickel and iron may be a single nitrogen and / or multiple nitrogens (e.g., N—N).

[0051] Meanwhile, according to an exemplary embodiment, the respective contents of nickel (Ni) and iron (Fe) in the catalyst may be, for example, about 0.1 to 10 wt % (preferably about 0.2 to 8 wt %, more preferably about 0.4 to 5 wt %, and particularly preferably about 0.7 to 1.5 wt %) and about 0.1 to 10 wt % (preferably about 0.2 to 8 wt %, more preferably about 0.4 to 5 wt %, and particularly preferably about 0.6 to 1.3 wt %).

[0052] The relative amounts of nickel (Ni) and iron (Fe) supported in the catalyst may be determined in consideration of the degree of interaction between them. For example, the atomic ratio of nickel (Ni) to iron (Fe) can be adjusted within the range of 1:about 0.4-1.6, preferably 1:about 0.6-1.4, and more preferably 1:about 0.8-1.2.

[0053] Method for producing a catalyst for carbon dioxide conversion Another embodiment of the present disclosure provides a method for preparing the aforementioned nickel (Ni)-iron (Fe) bimetallic catalyst for carbon dioxide conversion. In this regard, Figure 1 illustrates a schematic diagram of an exemplary embodiment of a method for preparing the bimetallic catalyst, specifically, by ion adsorption.

[0054] Referring to the figure, first, carbon (or carbon particles) is provided.

[0055] In the illustrated embodiment, the carbon is preferably selected from types that have good conductivity and properties that allow easy introduction of functional groups. According to an exemplary embodiment, the carbon may be at least one selected from, for example, carbon black, graphene, carbon nanotubes (CNTs), fullerenes, etc. However, from an economical perspective, it is preferable to use carbon black. The above types are understood to be illustrative and are not necessarily limited to these.

[0056] In this regard, carbon can be produced by methods known in the art or purchased from a commercial source. For example, organic compounds can be converted into carbon through a carbonization process (specifically, pyrolysis). Examples of such organic compounds include polymers (e.g., thermoplastic resins) and low-molecular-weight organic compounds, which can be used alone or in combination. Biomass can also be used. Exemplary carbon sources include at least one selected from conductive polymers, various petroleum products, coal, graphite, acetylene gas, and the like. The carbonization process described above is well known in the art, and further detailed description thereof will be omitted.

[0057] Meanwhile, according to an exemplary embodiment, the shape of the carbon (or carbon particles) may be, for example, spherical, elliptical, rod-like, plate-like, etc., and more specifically, may be in the form of a spherical powder. Furthermore, in the illustrated embodiment, the carbon may have nanoscale dimensions, and therefore its average particle size may be, for example, in the range of about 10 to 200 nm, preferably about 20 to 150 nm, and more preferably about 30 to 80 nm. In this case, the average particle size of the particles can be measured using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0058] According to an exemplary embodiment, carbon may advantageously have good conductivity to conduct an effective electrochemical reaction, for example, a conductivity of at least about 1.5 Scm. -1 , preferably at least about 2 Scm -1 , more preferably at least about 2.5 Scm -1 , particularly preferably at least about 2.7 Scm -1 may be.

[0059] Referring again to FIG. 1 , once carbon (or carbon particles) is provided, a step of forming an iron (Fe)-containing polymer / carbon composite may be performed by in-situ polymerization of a nitrogen-containing monomer on the carbon via chemical oxidation using an iron-based compound as an oxidant. However, according to an exemplary embodiment, prior to the polymerization reaction, impurities (e.g., inorganic impurities) and metals (e.g., alkali metals, alkali metal oxides, alkaline earth metals, and / or alkaline earth metal oxides) that may be contained in the carbon may clog the pore structure within the carbon. To remove these impurities, an acid washing or acid leaching may be optionally performed. The acid may be at least one selected from sulfuric acid, nitric acid, phosphoric acid, hydrochloric acid, formic acid, etc., and more specifically, sulfuric acid and / or hydrochloric acid. The acid washing may be performed in the form of an aqueous acid solution, and the concentration may be adjusted to, for example, about 1 to 8 M, preferably about 3 to 7 M, but this is to be understood as an exemplary purpose.

[0060] According to the illustrated embodiment, iron (Fe) can be introduced by adding a monomeric nitrogen source (nitrogen-containing monomer) to carbon (specifically, a carbon dispersion) and polymerizing it in situ in the presence of an iron-based oxidant, thereby contacting the polymeric nitrogen source with the carbon and adhering (or coating) it (forming an iron (Fe)-containing polymer / carbon composite).

[0061] In this regard, carbon may be dispersed in a liquid medium (dispersion medium) and then a nitrogen source (i.e., a nitrogen-containing monomer) may be added. The liquid medium may be at least one selected from water, aliphatic alcohols having 1 to 4 carbon atoms (e.g., methanol, ethanol, propanol (e.g., n-propanol and / or isopropanol), and / or n-butanol), and specifically, propanol. According to certain embodiments, a mixed solvent (or mixed solution) of alcohol and water may be used as the liquid medium. The volume ratio of alcohol to water may be adjusted within the range of, for example, about 1.5 to 5:1, preferably about 2 to 4:1, and more preferably about 2.5 to 3.5:1, but this is merely an example.

[0062] According to exemplary embodiments, the nitrogen-containing monomer added to the carbon dispersion is not limited to a specific compound, and one or more precursors may be used, as long as the compound contains both nitrogen and carbon and can be carbonized by a heat treatment process such as pyrolysis after polymerization. In certain embodiments, examples of the nitrogen-containing monomer include urea, melamine, ammonia, adenine, pyrrole, acrylonitrile, phenanthroline, pyrazole, vinylpyridine, pyrimidine, piperazine, pyran, carbamide, morpholine, imidazole, 1-methylimidazole, 2-methylimidazole, quinoxaline, aniline, benzimidazole, ethylenediamine, and cyanamide. According to certain embodiments, pyrrole may be used as the nitrogen-containing monomer. Because pyrrole monomer contains one nitrogen atom per molecule, it is advantageous in that it can effectively dope carbon with nitrogen through a polymerization reaction and also uniformly dope the carbon surface with nitrogen.

[0063] In this regard, the amount of nitrogen-containing monomer added to the carbon dispersion may be determined based on the amount of nitrogen ultimately doped into the carbon (i.e., the nitrogen doping amount), but since not all of the added nitrogen source is doped, an amount in excess of the actual amount may be added. For example, the amount of nitrogen-containing monomer relative to carbon may be about 0.01 to 1, preferably about 0.05 to 0.8, and more preferably about 0.1 to 0.4, by weight.

[0064] For example, the nitrogen-containing monomer may be added in a form dispersed in water, and in this case, the concentration of the nitrogen-containing monomer in the dispersion may be adjusted, for example, in the range of about 0.5 to 5 mmol, preferably about 1 to 4 mmol, more preferably about 1.5 to 3 mmol, which is understood as an illustrative example.

[0065] Meanwhile, according to exemplary embodiments, the iron-based oxidizing agent may be selected from a group that satisfies the requirement of high solubility in water or alcohol. Typically, an Fe(II) oxidizing agent and / or an Fe(III) oxidizing agent may be used. For example, the Fe(II) oxidizing agent (compound) may be at least one selected from the group consisting of Fe(NO), FeSO, Fe(acac), Fe(tfac), Fe(OAc), FeCl, FeBr, and FeI, and hydrates thereof. The Fe(III) oxidizing agent (compound) may be at least one selected from the group consisting of Fe(NO), Fe(SO), Fe(acac), Fe(tfac), Fe(OAc), Fe(OTs), Fe(OTf), FeCl, FeBr, and FeI, and hydrates thereof. According to certain embodiments, FeCl·6H0 may be used as the iron-based oxidizing agent.

[0066] The amount of the iron-based oxidant used may be, for example, about 0.2 to 5 times, preferably about 0.6 to 4 times, and more preferably about 1 to 3 times, based on the moles of the monomer. Furthermore, for efficient polymerization, the iron-based oxidant may be prepared in the form of a solution (e.g., an aqueous solution) and then added to the carbon dispersion containing the nitrogen-containing monomer. The concentration of the iron-based oxidant solution may be adjusted, for example, within the range of about 1 to 10 mmol, preferably about 2 to 7 mmol, and more preferably about 3 to 5 mmol, but this is to be understood as an example.

[0067] According to an exemplary embodiment, the polymerization reaction temperature is not particularly limited and may be adjusted typically within the range of about 5 to 60° C., preferably about 10 to 50° C., and more preferably about 20 to 40° C. The polymerization time may be, for example, about 0.1 to 30 hours, preferably about 1 to 20 hours, and more preferably about 1.5 to 10 hours.

[0068] Once the iron (Fe)-containing polymer / carbon composite is formed, a nickel precursor can be attached or introduced thereto to form a NiFe-polymer / carbon composite. In this regard, the nickel precursor may be introduced into the composite by a known impregnation method, such as impregnation (e.g., incipient wet impregnation), precipitation, strong reducing agent reduction, or polyol process. The basic principles of the above-mentioned impregnation methods are known in the art, and therefore, further explanation is omitted.

[0069] In an exemplary embodiment, the nickel precursor may be introduced into the iron (Fe)-containing polymer / carbon composite by ion adsorption and is typically used in the form of a water-soluble compound, such as a halide, organic acid salt, inorganic acid salt, hydroxide, complex, combination thereof, or a hydrate of the above-mentioned compound. For example, the inorganic acid salt may be at least one selected from nitrates, sulfates, carbonates, halides, etc. Furthermore, the organic acid salt may be at least one selected from acetates, alkoxides, glutarates, etc. Furthermore, the halide may be at least one selected from fluorides, chlorides, bromides, iodides, etc.

[0070] According to an exemplary embodiment, a nickel compound (specifically, a salt or complex) having an oxidation value of 2+ can be used as the nickel precursor. For example, the nickel compound can be at least one selected from nickel nitrate, nickel sulfate, nickel chloride, nickel carbonate, nickel acetylacetonate complex, nickel acetate, etc., or a hydrate thereof. Specifically, nickel nitrate and / or a hydrate thereof (e.g., Ni(NO3)2·6H2O) can be used.

[0071] According to an exemplary embodiment, the nickel precursor may be provided in the form of a solution. In this case, the solvent may be at least one selected from water, aliphatic alcohols having 1 to 4 carbon atoms (e.g., methanol, ethanol, propanol (e.g., n-propanol and / or isopropanol), and / or n-butanol), and specifically, propanol may be used. According to a specific embodiment, a mixed solvent of alcohol and water may be used as the solvent. In this case, the volume ratio of alcohol to water may be adjusted within a range of, for example, about 1.5 to 5:1, preferably about 2 to 4:1, and more preferably about 2.5 to 3.5:1, but this is understood as an exemplary purpose.

[0072] Furthermore, the concentration of the nickel precursor in the nickel precursor solution is not particularly limited, but can be adjusted in the range of, for example, about 0.05 to 0.5 mmol, preferably about 0.08 to 0.3 mmol, and more preferably about 0.1 to 0.2 mmol, taking into consideration the degree of ion adsorption, but this is understood as an illustrative example.

[0073] According to an exemplary embodiment, an iron (Fe)-containing polymer / carbon composite can be added to the nickel precursor solution prepared as described above. Alternatively, a dispersion of the iron (Fe)-containing polymer / carbon composite (e.g., a mixed liquid medium of alcohol and water) can be prepared, and then the nickel precursor or its solution can be added to the dispersion. According to an exemplary embodiment, the weight ratio of the iron (Fe)-containing polymer / carbon composite to the nickel precursor may depend on the amount of nickel supported in the final catalyst. Taking into account the amount of nickel that is not actually supported, the weight ratio may be adjusted within a range of, for example, 1: about 0.001 to 0.1, preferably 1: about 0.005 to 0.06, and more preferably 1: about 0.01 to 0.03, but this is understood as an exemplary purpose.

[0074] Referring again to FIG. 1, a pyrolysis step may be performed to form a bimetallic catalyst in which iron and nickel are each supported in monoatomic form from the NiFe-polymer / carbon composite formed by the introduction of the nickel precursor.

[0075] According to an exemplary embodiment, the pyrolysis may be carried out in an inert atmosphere, which may be formed by at least one gas selected from argon, nitrogen, helium, etc., and more specifically, may be carried out in a nitrogen atmosphere. The pyrolysis temperature may be adjusted to, for example, a temperature above about 700°C, preferably about 720 to 1000°C, and more preferably about 750 to 900°C. The heating rate during the pyrolysis step may be adjusted taking into account the degree of aggregation of metal ions, and may be, for example, about 0.5 to 8°C / min, preferably about 1 to 6°C / min, and more preferably about 2 to 4°C / min.

[0076] In this embodiment, the polymer / carbon composite is converted into a nitrogen-doped carbon (NC) nanostructure by pyrolysis, and the metal components, i.e., nickel (Ni) and iron (Fe), are bonded (coordinated or chelated) with the nitrogen atoms doped in the nitrogen-doped carbon nanostructure, which serves as a support, to form Ni-N4 sites and Fe-N5 sites, respectively, thereby providing catalytically active sites.

[0077] The bond or connection form between the Ni-N4 site and the Fe-N5 site may be affected by the pyrolysis temperature. Therefore, as described below, the pyrolysis temperature can be adjusted so that the nickel (Ni) at the Ni-N4 site and the iron (Fe) at the Fe-N5 site are not directly connected to each other but are indirectly connected via nitrogen while adjacent to each other. However, the pyrolysis temperature for realizing the desired metal arrangement or connection form may not be fixed or set to a specific temperature level, but may be changed or adjusted depending on the properties of the nitrogen-doped carbon nanostructure to be formed, the carbon structure, the nitrogen content, the chemical structure of the nitrogen source, etc. At this time, if the pyrolysis temperature is below a specific level, the nickel (Ni) and iron (Fe) may be directly connected, and therefore the connection form between the nickel and iron in the catalyst can be changed by adjusting the pyrolysis temperature.

[0078] On the other hand, the pyrolysis reaction time may affect the coordination number of nickel and iron with nitrogen, carbon conductivity, the type and content of functional groups, etc., and therefore, taking this into consideration, the reaction time may be adjusted, for example, to a maximum of about 8 hours, preferably about 0.1 to 3 hours, and more preferably about 0.5 to 2 hours.

[0079] As in the illustrated embodiment, a nitrogen-containing polymer in which iron is introduced into carbon is formed by oxidative polymerization using an iron-based oxidant, and then a nickel precursor is deposited and then thermally decomposed to form a nickel (Ni)-iron (Fe) / NC bimetallic catalyst. This method has the advantage that the iron precursor acts as an oxidant and is simultaneously doped, eliminating the need for an additional oxidant.

[0080] According to an exemplary embodiment, after pyrolysis, acid washing or acid leaching can be selectively performed to remove metals (Ni and / or Fe) that are not bonded to nitrogen atoms and metal particles that have aggregated at high temperatures, while distributing nickel and iron in a monoatomic form within the nitrogen-doped carbon nanostructures. The acid used in the acid washing step can be used. However, the acid concentration can be adjusted to a lower level, for example, about 0.3 to 2 M, preferably about 0.5 to 1.5 M, and more preferably about 0.8 to 1.2 M, but this is for illustrative purposes only. After pyrolysis or pyrolysis and acid washing, a bimetallic catalyst can be obtained by performing conventional post-treatments, such as at least one of water washing and drying. Such post-treatments are widely known in the art, and further detailed description will be omitted.

[0081] Meanwhile, in the nickel-iron diatomic metal catalyst according to this embodiment, the nickel and iron bonded to nitrogen exist adjacent to each other in the nitrogen-doped carbon nanostructure and are indirectly linked through the nitrogen, which may have a significant effect on the electronic structure between them. As a result, it is believed that low overpotential, high current density, and high carbon monoxide selectivity can be achieved by controlling the adsorption strength with the carbon dioxide conversion intermediate.

[0082] In this regard, the carbon dioxide uptake capacity (CO2 uptake capacity; 25°C) of the single atom catalyst may be, for example, at least about 1.5 μmol / g, preferably about 1-4 μmol / g, and more preferably in the range of about 2-3 μmol / g.

[0083] Electrodes (catalytic electrodes) and systems for carbon dioxide reduction reactions According to another embodiment of the present disclosure, a diatomic transition metal catalyst of nickel (Ni) and iron (Fe) may be loaded onto an electrochemical electrode and used to implement a carbon dioxide conversion system (specifically, a system for selectively reducing carbon dioxide to carbon monoxide). In particular, the catalyst-loaded electrode may be used as a cathode (reducing electrode) in the carbon dioxide conversion system, where carbon monoxide is produced.

[0084] According to an exemplary embodiment, during the manufacture of an electrode, a catalyst may be loaded onto a conductive substrate. The loading method is not particularly limited, and methods known in the art, such as loading using a slurry, vapor deposition, spray coating, etc., may be applied.

[0085] For example, in the slurry loading method, a slurry of the previously prepared catalyst (the dispersion medium may be at least one selected from the group consisting of alcohols having 1 to 4 carbon atoms (e.g., methanol, ethanol, propanol, etc.) and water (e.g., distilled water)) can be prepared. In this case, the concentration of the catalyst slurry may be adjusted, for example, within a range of about 5 to 30 wt %, preferably about 10 to 20 wt %. Thereafter, the slurry is applied to an electrode substrate (e.g., a conductive substrate) and then dried to prepare a catalyst electrode.

[0086] According to another embodiment, a catalyst can be loaded onto a substrate using a deposition method, and at this time, the catalyst layer to be loaded can be adjusted by adjusting the deposition rate, drying temperature, etc., and cleaning can be performed after deposition.

[0087] According to still another embodiment, spray coating can be applied, in which the coating liquid is prepared in the form of a mist and then applied to the substrate. Examples of such methods include an air spray method (using compressed air to prepare the coating liquid in the form of a mist and then spraying it onto the substrate), an ultrasonic spray method (using ultrasonic waves to form the coating liquid in the form of droplets and then apply it), and an electrostatic spray method (applying a high voltage to charge the coating liquid inside the nozzle, and then forming a mist by the repulsive force of the charge and applying it).

[0088] Meanwhile, according to an exemplary embodiment, the substrate on which the carbon dioxide conversion catalyst is loaded may have various shapes such as a plate, a rod, a mesh, a disk, a wire, paper, etc. In addition, the conductive substrate may be made of a material that can maintain conductivity even when exposed to an oxidizing atmosphere, such as at least one material selected from valve metals (specifically, titanium, aluminum, chromium, etc.), stainless steel, etc. (including alloys), carbon, etc.

[0089] In an exemplary embodiment, the amount of catalyst loaded is, for example, about 0.2 to 5 mg / cm on an electrode basis. 2 , preferably about 0.5 to 3 mg / cm 2 , more preferably about 0.8 to 2 mg / cm 2 , but this is understood to be for illustrative purposes only.

[0090] According to one embodiment of the present disclosure, a carbon dioxide conversion system can be realized in which the electrochemical electrode described above is used as a cathode to reduce carbon dioxide, generating carbon monoxide at the cathode and oxygen at the anode. Specifically, when a voltage is applied from an external power source, electrons are supplied to the cathode, which react with carbon dioxide at the cathode side to generate carbon monoxide gas and hydroxide ions (OH - ) is generated. At this time, the carbon monoxide gas generated is discharged to the outside, and the hydroxide ions move to the anode. The hydroxide ions that move to the anode may lose electrons (be oxidized) and be converted into oxygen gas, which may then be discharged.

[0091] In this regard, a carbon dioxide conversion system basically includes a pair of opposing electrodes (i.e., an anode and a cathode), each of which is electrically connected to an external power source (e.g., a potentiostat, a battery, etc.). Additionally, a three-electrode system may further include a reference electrode. Such a reference electrode may be selected from the group consisting of Ag / AgCl, a saturated calomel electrode (SCE), Hg / HgO, and Hg / Hg2SO4, and specifically, Ag / AgCl (3M NaCl) may be used.

[0092] According to an exemplary embodiment, the carbon dioxide conversion system uses, as a cathode, an electrochemical electrode containing or having the above-described catalyst attached thereto, as a pair of electrodes, and the remaining electrode, the anode, can be of any type known in the art. In this regard, the anode may also be made of a conductive material. For example, a conductive material applicable to the cathode may be selected from materials that easily generate carbon monoxide, are not decomposed upon contact with an electrolyte (e.g., an aqueous electrolyte), and exhibit particularly low overvoltage, such as at least one material (including alloys) selected from nickel, carbon, titanium, stainless steel, etc. In certain embodiments of the electrochemical carbon dioxide conversion system, each of the cathode and anode can be operated in full contact with or immersed in an electrolyte-containing aqueous solution.

[0093] Meanwhile, according to an exemplary embodiment, an electrolyte aqueous solution (which may be alkaline) in which an electrolyte is dissolved in an aqueous medium may be supplied to the carbon dioxide conversion system. In this case, the electrolyte may be at least one selected from potassium hydroxide, potassium bicarbonate, potassium bicarbonate, sodium bicarbonate, sodium hydroxide, lithium hydroxide, etc., and specifically may be potassium bicarbonate. Exemplary electrolyte concentrations may be, for example, in the range of about 0.05 to 3 M, preferably about 0.07 to 2 M, and more preferably about 0.1 to 1 M. Furthermore, the pH of the electrolyte-containing aqueous solution may be adjusted to a neutral or basic range, for example, in the range of about 6.5 to 14, preferably about 7 to 14, and more preferably about 8 to 14.

[0094] Alternatively, a system can be constructed using a polymer electrolyte membrane or the like as a separator. Such a separator is typically an ion exchange membrane (e.g., an anion exchange membrane) located between the cathode and the anode to separate the anode compartment from the cathode compartment. In this regard, the separator material may be selected from porous ceramic membranes (e.g., zirconia-based membranes), porous polymer membranes (e.g., polyolefin-based, more specifically, polypropylene-based membranes), solid polymer electrolyte membranes (ion exchange membranes made of perfluorosulfonic acid polymer materials such as Nafion, a trade name), and the like.

[0095] Generally, under thermodynamic standard conditions, the reduction reaction of carbon dioxide occurs near 0 V (vs. RHE). However, considering the concentration range of hydrogen ions actually participating in the reaction, it occurs at a negative potential. Furthermore, considering the reaction rate, a higher overpotential is required to electrochemically reduce carbon dioxide. Furthermore, it is preferable for the reduction reaction of carbon dioxide to suppress the hydrogen evolution reaction (HER), which occurs in a similar reduction potential range. The potentials at which currents of the same intensity flow at such a pair of opposing electrodes are different from each other, and the voltages at this time correspond to the overpotentials at each electrode.

[0096] In a carbon dioxide conversion system including an electrochemical electrode using the composite catalyst according to this embodiment, a reduction reaction can be carried out at a low overpotential. For example, when producing carbon monoxide from the cathode, operation at a low overpotential is possible. The carbon dioxide reduction reaction may be carried out at a temperature of, for example, about 15 to 40°C, preferably about 20 to 30°C, and more preferably at room temperature.

[0097] When the electrochemical electrode loaded with the single-atom catalyst according to this embodiment is used as a cathode, for example, an aqueous electrolyte solution of 0.1 M KHCO (pH 8.3) and a current of 200 mA / cm 2 Under current density conditions, the cell voltage may be, for example, about 3.3 V or less, preferably about 3.2 V or less, more preferably about 3 V or less, and particularly preferably about 2.88 V or less.

[0098] Meanwhile, the faradaic efficiency (FE) is an index for evaluating the efficiency and selectivity of an electrochemical reaction, and may be expressed by the following formula 1.

[0099]

number

[0100] where z is the number of electrons, n is the molar ratio of the products, I is the applied current, t is the reaction duration, and F is the Faraday constant.

[0101] In this regard, under the conditions described above, the faradaic efficiency (FE) may be at least about 90%, preferably at least about 93%, more preferably at least about 95%, and especially preferably at least about 97%.

[0102] Hereinafter, preferred examples will be presented to aid in understanding the present invention. However, the following examples are provided only to facilitate understanding of the present invention, and the present invention is not limited thereto.

[0103] Example Details regarding the materials used in this example, catalyst analysis, and electrochemical performance tests are as follows.

[0104] A. Substances - The Vulcan XC-72R Carbon was purchased from the Fuel cell store. Pyrrole (C4H5N) monomer, nitric acid, sulfuric acid, potassium bicarbonate, propanol, and nickel nitrate hexahydrate were each purchased from Sigma-Aldrich. Iron(III) chloride hexahydrate was purchased from Alfa Aesar. - Sigracet 39BB carbon paper and commercial Ag / C electrodes were purchased from Dioxide Materials. Deionized water (DI, 18 MΩ cm) was purchased from Millipore and used throughout the experiment.

[0105] B. Physicochemical catalyst analysis - High-resolution transmission electron microscopy (HR-TEM) and elemental mapping analysis were performed using a JEM-2100F equipped with a HAADF-STEM detector. - X-ray diffraction (XRD) analysis (PANalytical) was used to confirm the metal crystal plane. Measurements were taken using Cu Kα radiation at 40 kV and 100 mA, with a scan rate of 6° per minute at 0.01° intervals, within the range of 10 to 80°. - Using X-ray absorption fine structure (XAFS; PLS-II) analysis, we obtained Ni K-edge XAFS spectra in the photon energy range of 8132-9078 eV and Fe K-edge XAFS spectra in the photon energy range of 6910-8082 eV. The metal content (wt%) of the prepared catalyst was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, NexION 300X). - XPS spectra of surface elements were obtained using a high-performance X-ray photoelectron spectroscopy (XPS; VG multilab 2000). - Using Brunauer-Emmett-Teller analysis (Quadrasorb SI), the N2 sorption isotherm was measured at -196°C, and the specific surface area (m 2 / g) was calculated.

[0106] C. Electrochemical experiments Using a spray gun, 1 mg / cm was applied to carbon paper (Sigracet 39BB, Dioxide Materials). 2 The catalyst was coated at a loading of 1000 kJ / cm2 to prepare a reductive electrode. A commercial IrO2 catalyst and a Sustainion® anion exchange membrane (Dioxide Materials) were used as the oxidative electrode and the separator, respectively. The separator was positioned between the oxidative electrode and the reductive electrode, and the active area was 5 cm2. 2 A membrane electrode assembly (MEA) of a certain size was formed, and finally an electrochemical cell was assembled.

[0107] The electrolyte used was a 0.1 M KHCO3 solution (neutral pH), which was circulated at a rate of 3 mL / min on the oxidizing electrode side, and carbon dioxide gas was continuously supplied to the reducing electrode side at a flow rate of 30 mL / min.

[0108] Electrochemical carbon dioxide conversion performance was measured using chronopotentiometry at 200 mA / cm 2The analysis was carried out under the conditions above, and the generated gas was analyzed by gas chromatography (Agilent 7890).

[0109] Example 1 Fabrication of NiFe-DAC Vulcan XC-72R Carbon (Fuel Cell Store) was treated with a mixture of 6M HNO3 and 6M H2SO4 for one day and then dried. 400 mg of dried carbon was dispersed in a mixture of propanol (Sigma) and distilled water (3:1 volume ratio), and then pyrrole monomer (2.2 mmol) dispersed in 10 mL of distilled water was added and stirred for 30 minutes to prepare a suspension.

[0110] Then, ferric chloride hexahydrate (Iron(III) chloride hexahydrate; 4.4 mmol) was dissolved in 10 mL of distilled water, and this was added to the above suspension and stirred for 4 hours to produce an Fe-doped polypyrrole-carbon composite (Fe-PPyC).

[0111] Next, NiFe-DAC was fabricated using incipient wetness impregnation. Specifically, 150 mg of the previously synthesized Fe-PPyC was added to a metal precursor solution prepared by dissolving 0.11 mmol of nickel nitrate hexahydrate in 4 mL of propanol / distilled water (3:1 volume ratio). The solution was then sonicated for 2 hours and dried. The resulting mixture was then pyrolyzed at 800°C for 1 hour under a N2 atmosphere. Finally, the mixture was washed with a 1 M H2SO4 solution to remove impurities, yielding NiFe-DAC (diatomic catalyst).

[0112] Comparative Example 1 Fe-SAC production A catalyst was produced by the same procedure as in Example 1, except that Fe-PPyC was pyrolyzed without using nickel nitrate hexahydrate, to obtain Fe-SAC (single atomic catalyst).

[0113] Comparative Example 2 Manufacturing of NiFe-DAC (Direct) NiFe-DAC(Direct) was obtained by preparing a catalyst according to the same procedure as in Example 1, except that the pyrolysis temperature under N2 atmosphere was set at 700°C.

[0114] Comparative Example 3 A commercial reducing electrode catalyst, Ag / C (Dioxide Materials), was prepared.

[0115] Catalyst characterization - HADDF-STEM analysis The NiFe-DAC catalyst prepared in Example 1 was subjected to HADDF-STEM analysis, and the results are shown in FIGS. 2A to 2C.

[0116] Referring to the figure, it was confirmed that the catalyst according to Example 1 has a diatomic structure in which Ni and Fe are spaced apart by an average of about 0.27 nm, and in particular, it was confirmed that 65% of the observed single atoms exist in pairs.

[0117] -XRD analysis The XRD patterns of the catalyst (NiFe-DAC) according to Example 1 and the catalysts (Fe-SAC and NiFe-DAC(Direct)) according to Comparative Example 1 are shown in FIG.

[0118] Referring to the figure, no peaks associated with other metals were observed except for the carbon peak (002 plane) around 22° in either the catalysts prepared in Example 1 or Comparative Example 1. This result indicates that the metal is uniformly distributed in a monoatomic state on the carbon support without the formation of metal crystalline particles.

[0119] - XPS analysis The catalyst (NiFe-DAC) according to Example 1 and the catalyst (Fe-SAC) according to Comparative Example 1 were each subjected to XPS analysis, and the results are shown in FIG.

[0120] Referring to the figure, Ni 2p and Fe 2p peaks are observed in the catalyst (NiFe-DAC) according to Example 1, which indicates that the active metals Ni and Fe are simultaneously present on the carbon support.

[0121] On the other hand, only the Fe 2p peak was observed in the catalyst (Fe-SAC) according to Comparative Example 1, indicating that only Fe was present as the active metal in the catalyst. Furthermore, in the case of Fe in NiFe DAC, the Fe 2p peak shifted to a higher binding energy, confirming that the introduction of Ni changed the electronic structure.

[0122] - Analysis of metal content in catalysts The catalysts prepared in Example 1 and Comparative Example 1 were subjected to ICP analysis for metals (Ni and / or Fe), and the results are shown in Table 1 below.

[0123] [Table 1]

[0124] Referring to the above table, it can be seen that Ni and Fe are introduced into the catalyst (NiFe-DAC) according to Example 1.

[0125] - EXAFS analysis EXAFS analysis was carried out on the catalyst (NiFe-DAC) according to Example 1 and the catalyst (Fe-SAC) according to Comparative Example 1, and the results are shown in FIG. 5A.

[0126] Referring to the figure, the Fe K-edge Fourier-transformed (FT) EXAFS spectrum shows that both NiFe-DAC and Fe-SAC possess Fe-N peaks. This indicates that Fe exists in a monoatomic state bound to N. Furthermore, the Ni FT-EXAFS spectrum reveals that Ni in NiFe-DAC exists in a monoatomic state bound to N. EXAFS fitting reveals that Ni is bound to four N atoms, and Fe is bound to five N atoms.

[0127] On the other hand, EXAFS analysis was carried out on the catalyst (NiFe-DAC(Direct)) according to Comparative Example 2, and the results are shown in FIG. 5B.

[0128] Referring to the figure, in the catalyst of Comparative Example 2 (NiFe-DAC(Direct)), in addition to the Fe-N peak indicating a single atom state, a minor peak indicating an Fe-Ni bond was observed around 2.5 Å. The above results indicate that NiFe-DAC(Direct) also has an Fe-Ni bond, which affects the electronic structure and ultimately suggests that the efficiency of carbon monoxide production by carbon dioxide conversion is different from that of the catalyst of Example 1.

[0129] - BET analysis BET analysis was performed on the catalyst according to Example 1 (NiFe-DAC) and the catalyst according to Comparative Example 1 (Fe-SAC), and the results are shown in Figure 6. Referring to the figure, the catalysts produced according to Example 1 and Comparative Example 1 have similar pore structures, specific surface areas (approximately 148 to 152 m), and 2 The reason for the similar pore structure is believed to be that they were heat treated at the same temperature.

[0130] Electrochemical performance evaluation The faradaic efficiency (200 mA / cm) for carbon monoxide production was measured for each of the catalyst (NiFe-DAC) according to Example 1 and the catalysts (Fe-SAC, NiFe-DAC (Direct), and Ag / C) according to Comparative Examples 1 to 3. 2 The results are shown in Figure 7.

[0131] As shown in the figure, the NiFe-DAC catalyst showed the highest faradaic efficiency (FE) for carbon monoxide (CO) production at the lowest voltage compared to the single-atom catalysts Fe-SAC, NiFe-DAC (Direct), and commercial Ag / C.

[0132] Considering the above-mentioned physicochemical analysis results, it is believed that in the NiFe-DAC catalyst, the Ni-N4 site and the Fe-N5 site are adjacent to each other, inducing a synergistic effect due to strong electrical interaction, thereby appropriately controlling the electronic structure of the NiFe-DAC catalyst and, as a result, promoting the conversion reaction of carbon dioxide to carbon monoxide.

[0133] Simple variations or modifications of the present invention may be readily utilized by those having ordinary skill in the art, and all such variations or modifications may be considered to fall within the scope of the present invention.

Claims

1. (i) a support comprising nitrogen-doped carbon nanostructures ((N-C) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; Including, Nickel (Ni) is bonded to each of the four surrounding nitrogen atoms, while iron (Fe) is bonded to each of the five surrounding nitrogen atoms, forming Ni—N ４ site and Fe—N ５ Create a site, and Ni-N ４ Ni and Fe-N sites ５ An electrochemical catalyst in which the Fe atoms at the sites are indirectly connected to each other through nitrogen atoms.

2. 2. The catalyst according to claim 1, wherein the content of nickel (Ni) is in the range of 0.1 to 10 wt % and the content of iron (Fe) is in the range of 0.1 to 10 wt %.

3. The catalyst according to claim 2, wherein the atomic ratio of nickel (Ni) to iron (Fe) in the catalyst is 1:0.4 to 1.

6.

4. The catalyst is 100 to 300 m ２ / g specific surface area (BET) and 0.1-0.5 cm ３ Catalyst according to claim 1, characterized in that it exhibits a pore volume of 1000 .mu.m / g.

5. 2. The catalyst according to claim 1, wherein the nitrogen content in the nitrogen-doped carbon nanostructure is in the range of 1 to 12 atomic %.

6. a) forming an iron (Fe)-containing polymer / carbon composite by attaching a nitrogen-containing monomer to carbon and polymerizing in-situ using an iron-based oxidizing agent; b) depositing a nickel precursor onto the iron (Fe)-containing polymer / carbon composite to form a NiFe-polymer / carbon composite; c) pyrolyzing the NiFe-polymer / carbon composite to deposit nickel (Ni) and iron (Fe) in monoatomic form on a support comprising nitrogen-doped carbon nanostructures ((N-C) nanostructures); Including, the nitrogen-containing monomer comprises pyrrole; The pyrolysis is carried out at a temperature of greater than 700°C and not greater than 1000°C for 0.1 to 8 hours; Nickel (Ni) is bonded to each of the four surrounding nitrogen atoms, while iron (Fe) is bonded to each of the five surrounding nitrogen atoms, forming Ni—N ４ site and Fe—N ５ Create a site, and Ni-N ４ Ni and Fe-N sites ５ A method for producing an electrochemical catalyst, wherein the Fe atoms at the sites are adjacent to each other and indirectly connected via nitrogen atoms.

7. 7. The method for preparing an electrochemical catalyst according to claim 6, wherein the carbon comprises at least one selected from the group consisting of carbon black, graphene, carbon nanotubes (CNTs), and fullerenes.

8. 7. The method for producing an electrochemical catalyst according to claim 6, wherein the carbon has nanoscale dimensions and an average particle size of 10 to 200 nm.

9. The carbon has a viscosity of at least 1.5 Scm －１ 7. The method for producing an electrochemical catalyst according to claim 6, wherein the catalyst exhibits a conductivity of 0.1 to 1.

0.

10. 7. The method for producing an electrochemical catalyst according to claim 6, wherein the iron-based oxidant is an Fe(II) oxidant and / or an Fe(III) oxidant.

11. The Fe(II) oxidant is Fe(NO ３ ) ２ , FeSO ４ , Fe(acac) ２ , Fe(tfac) ２ , Fe(OAc) ２ , FeCl ２ , FeBr ２ and FeI ２ and at least one selected from the group consisting of hydrates thereof, and The Fe(III) oxidant is Fe(NO ３ ) ３ , Fe ２ (SO ４ ) ３ , Fe(acac) ３ , Fe(tfac) ３ , Fe(OAc) ３ , Fe(OTs) ３ , Fe(OTf) ３ , FeCl ３ , FeBr ３ and FeI ３ 11. The method for producing an electrochemical catalyst according to claim 10, wherein the catalyst is at least one selected from the group consisting of:

12. 7. The method for producing an electrochemical catalyst according to claim 6, wherein the nickel precursor is at least one selected from the group consisting of nickel nitrate, nickel sulfate, nickel chloride, nickel carbonate, nickel acetylacetonate complex, nickel acetate, and hydrates thereof.

13. an electrode substrate; and a bimetallic catalyst loaded on the electrode substrate; An electrode for a carbon dioxide reduction reaction comprising: The diatomic metal catalyst is (i) a support comprising nitrogen-doped carbon nanostructures ((N-C) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; an electrochemical catalyst comprising In the bimetallic catalyst, nickel (Ni) is bonded to each of the four surrounding nitrogen atoms, while iron (Fe) is bonded to each of the five surrounding nitrogen atoms, forming Ni—N ４ site and Fe—N ５ Create a site, and Ni-N ４ Ni and Fe-N sites ５ An electrode in which the Fe atoms at the sites are indirectly connected to each other while being adjacent to each other via nitrogen.

14. The loading of the diatomic metal catalyst in the electrode is 0.2 to 5 mg / cm ２ 14. The electrode according to claim 13, wherein

15. an anode and a cathode as electrochemical electrodes electrically connected to an external power source; and an aqueous electrolyte disposed between the anode and the cathode; 1. A carbon dioxide reduction system comprising: When a voltage is applied from the external power supply, oxygen is generated from the anode side, and carbon monoxide is generated from the cathode side. the cathode comprises a bimetallic catalyst loaded on an electrode substrate; The diatomic metal catalyst is (i) a support comprising nitrogen-doped carbon nanostructures ((N-C) nanostructures); and (ii) nickel (Ni) and iron (Fe), each deposited in monoatomic form on said support; an electrochemical catalyst comprising In the bimetallic catalyst, nickel (Ni) is bonded to each of the four surrounding nitrogen atoms, while iron (Fe) is bonded to each of the five surrounding nitrogen atoms, forming Ni—N ４ site and Fe—N ５ Create a site, and Ni-N ４ Ni and Fe-N sites ５ The Fe atoms at the sites are indirectly connected to each other while being adjacent to each other via nitrogen atoms.

16. The electrolyte comprises at least one selected from the group consisting of potassium hydroxide, potassium bicarbonate, potassium bicarbonate, sodium bicarbonate, sodium hydroxide, and lithium hydroxide; and 16. The system of claim 15, wherein the pH of the aqueous electrolyte is controlled within a range of 6.5 to 14.

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