Monoatomic metal catalyst and carbon dioxide conversion system using the same

Abstract

To provide: a transition metal single-atom catalyst capable of improving price competitiveness while providing better carbon dioxide conversion activity than noble metal catalysts; and an electrochemical electrode including the same.SOLUTION: The invention provides: an electrochemical hybrid catalyst in which two transition metals in the form of single atoms are loaded adjacently to each other in a mutually indirectly linked form in a nitrogen-doped carbon nanostructure (N-C) composite, and thus exhibits high carbon monoxide selectivity and current density under a low overpotential condition during reduction reaction for converting carbon dioxide into carbon monoxide; and a carbon dioxide conversion system using the same.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a monoatomic metal catalyst 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 supporting two monoatomic transition metals adjacent to each other in an indirectly bonded form on a nitrogen-doped carbon nanostructure (NC) composite, and a 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 as the global population has grown due to the massive consumption of fossil fuels. 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 of the earth's surface and the lower atmosphere, causing global warming and climate change, and is emerging as a fatal threat to the ecosystem.

[0003] In response to this, countries around the world have announced plans to reduce greenhouse gases, increasing the burden of reducing carbon dioxide emissions across the industrial sector. While research has previously focused on carbon 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 goal of reducing greenhouse gas emissions by 37% compared to business as usual (BAU) levels by 2030, as specified in the 2015 Paris Climate Change Agreement.

[0004] Proposed carbon dioxide conversion technologies include electrochemical conversion, photochemical conversion, catalytic chemical conversion, and biological conversion. Among these, electrochemical conversion technology (i) can be operated under room temperature and pressure conditions, (ii) has a simple and modular system, (iii) can increase the net carbon dioxide reduction rate when utilizing renewable energy, and (iv) can be used as a renewable energy storage technology because it converts 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 a variety of organic compounds, minimizing pollutant emissions and being environmentally friendly. Thus, the high-value-added chemicals and fuels produced by 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, diverse product distribution (low selectivity), and the occurrence of competing reactions such as hydrogen evolution reaction (HER). To overcome these technological barriers, electrochemical catalysts with higher catalytic activity and efficiency for CO2 reduction are required. Existing research has suggested a variety of electrochemical CO2 conversion catalysts. Examples include non-metallic (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 produce excellent results in terms of current density and faradaic efficiency for CO.

[0006] However, because precious metal catalysts are disadvantageous in terms of commercial application due to the use of expensive precious metals, interest has recently been growing in single-atom catalysts (SACs), catalysts in which metals are dispersed (or supported) to the size of a single atom. Single-atom catalysts can maximize carbon dioxide conversion activity by reducing the size of the metal active site to the sub-nanoatom level, providing a solution for replacing expensive precious metals with inexpensive transition metals. In particular, their extremely simplified and ideal structure makes them ideal for replicating theoretical results obtained through computational science and conducting related research (e.g., J. Phys. Chem. C, 2013, 117, 9187-9195; Angew. Chem. Int. Ed., 2015, 54, 10758-10762). However, in the case of single-atom metal catalysts, the atoms tend to aggregate and form particles, making it difficult to commercially synthesize stable single-atom catalysts. The costly and complicated synthesis process hampers their application in various fields. In particular, the structural simplicity of single-atom active sites limits their ability to enhance the catalytic efficiency of the carbon dioxide reduction process, which involves multiple proton-electron transfer steps.

[0007] In response to this, carbon-based metal-nitrogen (MN) catalysts have been developed that provide single-atom transition metal active sites. These catalysts have low cost, adjustable structure, high activity, and safety, and 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 substitutes for 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 a technique to adjust the electronic structure of single-atom catalysts to improve the carbon dioxide conversion performance of existing single-atom catalysts. 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 specific example of the present disclosure provides a carbon dioxide conversion system that can effectively remove carbon dioxide from exhaust gases emitted from power plants, steel mills, etc., by applying a catalyst that has higher carbon dioxide conversion activity and higher selectivity for carbon monoxide than conventional technologies. [Means for solving the problem]

[0012] According to the first aspect of the present disclosure, nitrogen-doped carbon nanostructures; A first transition metal (M1) and a second transition metal (M2) in monatomic form supported on the nitrogen-doped carbon nanostructure; An electrochemical catalyst comprising: The catalyst is a M1-N catalyst in which a first transition metal (M1) and a second transition metal (M2) are bonded to nitrogen atoms of a nitrogen-doped carbon nanostructure. x Moiety and M2-N x Moiety (N x means a nitrogen atom having coordinating ability), A catalyst is provided in which M1 and M2 are indirectly linked.

[0013] According to an exemplary embodiment, the first transition metal (M1) and the second transition metal (M2) in the catalyst are M1-N x Moiety and M2-N x Moiety is M1-N x -N x -M2.

[0014] According to an illustrative embodiment, the first transition metal (M1) may be nickel (Ni) and the second transition metal (M2) may be manganese (Mn).

[0015] According to a second aspect of the disclosure: a) pyrolyzing a carbon precursor in an inert atmosphere at a first heat treatment temperature to prepare porous carbon; b) contacting the porous carbon with a nitrogen source to form a nitrogen source-porous carbon composite; c) adsorbing a precursor of a first transition metal (M1) and a precursor of a second transition metal (M2) on the nitrogen source-porous carbon composite; d) pyrolyzing the nitrogen source-porous carbon composite having the first transition metal (M1) and the second transition metal (M2) adsorbed thereon in an inert atmosphere at a second heat treatment temperature to support the first transition metal (M1) and the second transition metal (M2) in monatomic form on the nitrogen-doped carbon nanostructure; Including, M1-N, in which a first transition metal (M1) and a second transition metal (M2) are bonded to the nitrogen atoms of a nitrogen-doped carbon nanostructure, respectively. x Moiety and M2-N x Moiety (N x means a nitrogen atom having coordinating ability), A method for preparing an electrochemical catalyst is provided in which a first transition metal (M1) and a second transition metal (M2) are indirectly bonded.

[0016] According to the third aspect of this disclosure: an electrode substrate; a single-atom metal catalyst loaded on the electrode substrate; An electrode for a carbon dioxide reduction reaction comprising: The monoatomic metal catalyst is nitrogen-doped carbon nanostructures; A first transition metal (M1) and a second transition metal (M2) in monatomic form supported on the nitrogen-doped carbon nanostructure; an electrochemical catalyst comprising The catalyst is a M1-N catalyst in which a first transition metal (M1) and a second transition metal (M2) are bonded to nitrogen atoms of a nitrogen-doped carbon nanostructure. x Moiety and M2-N x Moiety (N x means a nitrogen atom having coordinating ability), An electrode is provided in which the first transition metal (M1) and the second transition metal (M2) are indirectly bonded.

[0017] According to an exemplary embodiment, the loading of the bimetallic monoatomic metal catalyst in the electrode is 0.2 to 5 mg / cm. 2 The range may be:

[0018] According to the fourth aspect of this disclosure: an anode and a cathode as electrochemical electrodes electrically connected to an external power source; an aqueous electrolyte filled 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 on the anode side, while carbon monoxide is generated on the cathode side; the cathode comprises a monoatomic metal catalyst loaded on an electrode substrate; The monoatomic metal catalyst is nitrogen-doped carbon nanostructures; A first transition metal (M1) and a second transition metal (M2) in monatomic form supported on the nitrogen-doped carbon nanostructure; an electrochemical catalyst comprising The catalyst is a M1-N catalyst in which a first transition metal (M1) and a second transition metal (M2) are bonded to nitrogen atoms of a nitrogen-doped carbon nanostructure. x Moiety and M2-N x Moiety (N x means a nitrogen atom having coordinating ability), A system is provided in which the first transition metal (M1) and the second transition metal (M2) are indirectly bonded. [Effects of the Invention]

[0019] The transition metal monoatomic catalyst according to the present disclosure comprises two transition metals (first transition metal (M1) and second transition metal (M2)) bonded to nitrogen and supported adjacent to each other in a non-bonded manner on a nitrogen-doped carbon nanostructure. This allows the catalyst to achieve better carbon dioxide conversion performance (low overpotential, high current density, and high carbon monoxide selectivity) than monoatomic catalysts in which two transition metals are directly bonded, as well as monoatomic catalysts in which two transition metals are directly bonded. Furthermore, the catalyst is economically advantageous because it can provide improved electrochemical catalytic activity compared to commercial noble metal catalysts while using inexpensive transition metals or base metals in the carbon dioxide reduction reaction. Furthermore, the monoatomic metal catalyst according to the present disclosure can be prepared by a simple process, such as ion adsorption, and can be applied to the electrode (i.e., cathode) of an electrochemical system for carbon dioxide reduction to reduce carbon dioxide and effectively produce highly additive compounds (e.g., carbon monoxide, a component of synthesis gas). Therefore, the catalyst is expected to be widely used in the future. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram illustrating a process for preparing a single atom catalyst according to an example embodiment. [Figure 2] FIG. 1 is a diagram showing a series of processes for preparing a dual monatomic (Ni and Mn) catalyst (non-bonding NiMn / NPC and direct bonding NiMn / NPC) and a single metal monatomic metal catalyst (Ni / NPC and Mn / NPC) in Examples and Comparative Examples, respectively. [Figure 3] SEM photographs, TEM photographs, HAADF-STEM / element mapping photographs, intensity profiles, and corresponding EEL spectra (Electron Energy-Loss Spectroscopy) of the NiMn / NPC catalyst. [Figure 4]These are XRD (X-ray diffraction) patterns of the binary monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC) and the monometallic monatomic metal catalysts (Ni / NPC and Mn / NPC). [Figure 5] 1 is a graph showing the results of Brunauer-Emmett-Teller (BET) analysis of a binary monatomic (Ni and Mn) catalyst (non-bonding NiMn / NPC) and a single metal monatomic metal catalyst (Ni / NPC and Mn / NPC), respectively. [Figure 6] Figure 6a shows the results of synchrotron-based X-ray absorption near-edge structure (XANES) analysis of Ni phthalocyanine, Ni foil, bicomponent monoatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), and monometallic monoatomic metal catalyst (Ni / NPC). Figure 6b shows the results of synchrotron-based X-ray absorption near-edge structure (XANES) analysis of Mn phthalocyanine, Mn foil, bicomponent monoatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), and monometallic monoatomic metal catalyst (Mn / NPC). [Figure 7]Figure 7a shows the results of extended X-ray absorption fine structure (EXAFS) analysis of Ni phthalocyanine, Ni foil, bicomponent monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), and monometallic monatomic catalyst (Ni / NPC). Figure 7b shows the results of extended X-ray absorption fine structure (EXAFS) analysis of Mn phthalocyanine, Mn foil, bicomponent monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), and monometallic monatomic catalyst (Mn / NPC). [Figure 8] Figure 8a shows the Faradaic efficiency for carbon monoxide production for the bicomponent monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), the monometallic monatomic metal catalysts (Ni / NPC and Mn / NPC), and the commercial Ag / C catalyst. Figure 8b shows the current density for carbon monoxide production for the bicomponent monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC), the monometallic monatomic metal catalysts (Ni / NPC and Mn / NPC), and the commercial Ag / C catalyst. [Figure 9] Tafel plots for carbon monoxide production for binary monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC) and a commercial Ag / C catalyst, respectively. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention can be fully achieved by the following description. It should be understood that the following description describes preferred embodiments of the present invention, but the present invention is not necessarily limited thereto. 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 referring to the specific meaning of the related description below.

[0022] As used herein, the term "catalyst" may refer to a component that increases the rate of an electrochemical reaction, participates in the reaction itself, but is not consumed by the reaction itself, and can participate in the electrochemical reaction. In a narrower sense, the term may refer to a component that promotes the reaction mechanism of donating or accepting electrons in the carbon monoxide production reaction via a carbon dioxide reduction reaction and / or the oxygen production reaction.

[0023] The term "electrochemical carbon dioxide conversion reaction" refers generally to a reaction in which carbon dioxide is reduced to a useful compound 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, to a reaction in which carbon dioxide is reduced to carbon monoxide.

[0024] The term "nanostructure" may refer to any nanoscale object having a feature or texture having a dimension or size on the nanoscale (e.g., about 0.1 to 1000 nm, specifically 1 to 100 nm), such as a nanopillar, nanorod, nanowall, nanowire, nanoweb, etc.

[0025] A "single atom catalyst" can be understood as a catalyst in which the active metal, attached or fixed to a support, is present in isolated or individualized atomic form.

[0026] "Non-bonding" can mean that two atoms or ions are connected via another atom or group rather than being directly connected to each other. Conversely, "direct bonding" can mean that two atoms are directly connected to each other (e.g., metal-metal bonding).

[0027] As used herein, the term "electrode" refers to a conductive structure that is electrically connected to an external power source in a carbon dioxide reduction reaction (CORR) system and is typically externally separated from the external power source, and may refer to a cathode where a carbon dioxide reduction reaction occurs and an anode where an oxidation reaction occurs upon application of a voltage from the external power source.

[0028] A "catalytic electrode" may refer to a current collector loaded with a catalyst that is absorbed on or otherwise electrically connected to the current collector. Here, the catalyst may be understood to be capable of associating with the current collector through a change in oxidation state and / or dynamic equilibrium with the aqueous medium when exposed to an aqueous medium (e.g., an aqueous electrolyte solution).

[0029] "Electrochemical electrode" may refer to an electrode structure in which a catalytic component that promotes the carbon dioxide reduction reaction is incorporated into or deposited on the substrate of the electrode (cathode and / or anode).

[0030] "Overpotential" may refer to a voltage other than the thermodynamic reduction or oxidation voltage required to obtain a desired catalytic activity. As used herein, it may refer to the voltage that must be applied to an electrode to cause a carbon dioxide reduction reaction minus the thermodynamic voltage required for that reaction.

[0031] Terms such as "left side" or "right side" can be understood to describe the relative positional relationship between components or members, and terms such as "located on the left side" or "located on the right side" can be understood to express the relative positional relationship not only in contact with a particular object but also in a non-contact state.

[0032] In this specification, when a numerical range is specified by a lower limit and / or an upper limit, it can be understood that any subcombination within that numerical range is also disclosed. For example, when it is stated as "1 to 5," it includes 1, 2, 3, 4, and 5, as well as any subcombination therebetween.

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

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

[0035] When a term "comprises" a certain element, this means that it may further comprise other elements, unless otherwise specified.

[0036] Single-atom catalyst for carbon dioxide conversion An electrochemical catalyst according to an embodiment of the present disclosure can be used as a cathode catalyst in a carbon dioxide reduction system in which an anode or oxidation electrode and a cathode or reduction electrode are electrically connected to an external power source and an electrolyte (specifically, an aqueous electrolyte) is filled or loaded between the anode and cathode. Such an 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 as shown in Reaction Scheme 1 below, while an oxygen production reaction occurs via an oxidation reaction at the anode as shown in Reaction Scheme 2 below.

[0037] [Reaction Scheme 1] CO2+H2O+2e - →CO+2OH [Reaction Scheme 2] 2OH - →1 / 2O2+H2O+2e -

[0038] In particular, the detailed mechanism of the carbon dioxide reduction reaction at the cathode can be shown in Reaction Scheme 3 below.

[0039] [Reaction Scheme 3] [ka]

[0040] According to this embodiment, an electrochemical catalyst for promoting the carbon dioxide reduction reaction (CO2RR) occurring at the cathode is provided, in which two metals, i.e., a first transition metal (M1) and a second transition metal (M2), are supported on a nitrogen-doped carbon nanostructure in a monatomic form. The catalyst is a hybrid catalyst in which both transition metals coexist in a monatomic form, and in particular, the first transition metal (M1) and the second transition metal (M2) exist adjacent to each other in an atomic state (monatomic form). As a result, the carbon dioxide conversion mechanism according to Reaction 3 can effectively perform both carbon dioxide adsorption and activation, as well as efficient carbon monoxide desorption.

[0041] Furthermore, it is noteworthy that the two metals (first transition metal (M1) and second transition metal (M2)) are indirectly bonded (linked) to each other rather than directly bonded (linked) to each other, and therefore, a high carbon dioxide conversion rate and carbon monoxide selectivity can be achieved even under low overpotential conditions compared to not only single-metal monoatomic catalysts but also monoatomic catalysts in which two metals are directly bonded to each other.

[0042] According to one embodiment, the single-atom catalyst comprises two transition metals, specifically a first transition metal (M1) and a second transition metal (M2), each dispersed or supported in a single-atom form on a nitrogen-doped carbon nanostructure as a matrix.

[0043] Meanwhile, nitrogen-doped carbon nanostructures can be porous. Here, the average pore size (diameter) can be, for example, from about 50 nm to about 1 μm, specifically about 100 to 700 nm, more specifically about 200 to 500 nm. If the pore size is too small, the active sites may not be uniformly distributed, limiting the transport of reactants / products. On the other hand, if the pore size is too large, the electrode resistance may increase. Therefore, it is advantageous to adjust the pore size within the above-mentioned range, but this should be understood as an example. Furthermore, the pores in porous nitrogen-doped carbon nanostructures can exhibit hierarchical structural characteristics.

[0044] According to an illustrative example, the nitrogen content in the nitrogen-doped carbon nanostructure may be, for example, in the range of about 1 to 12 atomic %, specifically about 2 to 10 atomic %, and more specifically 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 advantageous to appropriately adjust it within the aforementioned range. However, the present disclosure is not limited thereto.

[0045] According to this specific example, each of the two transition metals (M1 and M2) can be selected from a group that can function as an active site during the carbon dioxide conversion reaction and / or can provide control over the electronic structure of the active site. Furthermore, the combination or pair of the two transition metals can be selected so that charge transfer occurs efficiently due to the difference in electronegativity between them. For example, the two transition metals can be combined so that the difference in electronegativity between them is, for example, about 0.1 to 0.5, specifically 0.2 to 0.4.

[0046] According to an exemplary embodiment, the first transition metal (M1) may be at least one selected from, for example, nickel (Ni), iron (Fe), silver (Ag), copper (Cu), cobalt (Co), zinc (Zn), etc. The second transition metal (M2) may be at least one selected from, for example, manganese (Mn), molybdenum (Mo), titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), zirconium (Zr), cadmium (Cd), etc.

[0047] In a specific example, the first transition metal (M1) can be nickel (Ni) and the second transition metal (M2) can be manganese (Mn). Therefore, the nickel (Ni)-manganese (Mn) binary combination is advantageous in that the nickel acts as an active site for carbon dioxide conversion, and the manganese effectively controls the electronic structure of the nickel, thereby improving the catalytic performance.

[0048] Additionally, according to exemplary embodiments, the single atom catalyst may optionally further comprise other metals or transition metals in addition to the combination of two transition metals for further modification or modification, in which case the additional metals may also be introduced in single atom form.

[0049] According to this specific example, each of the first transition metal (M1) and the second transition metal (M2) is bonded (specifically, coordinated) to the nitrogen atom of the nitrogen-doped carbon nanostructure, specifically M1-N x Moiety and M2-N x It can be contained in the catalyst as a moiety, which can function as an active site for the carbon dioxide reduction reaction. x can mean a nitrogen atom having coordinating ability. For example, x can be determined within the range of about 0.5 to 2.5, specifically about 0.8 to 2.3, and more specifically about 1 to 2, but this can be understood as an illustrative example.

[0050] According to exemplary embodiments, the first transition metal (M1) and the second transition metal (M2) can be indirectly bonded to each other, for example, via nitrogen (N). In certain embodiments, the M1-N bond in the catalyst can be x Moiety and M2-N x Moiety is M1-N x -N x -M2.

[0051] According to illustrative embodiments, the respective contents of the first transition metal (M1) and second transition metal (M2) in the catalyst can be determined, for example, in the ranges of about 0.1 to 1 wt % (specifically, about 0.2 to 0.8 wt %, more specifically, about 0.3 to 0.6 wt %) and about 0.05 to 0.8 wt % (specifically, about 0.1 to 0.7 wt %, more specifically, about 0.2 to 0.5 wt %).

[0052] Furthermore, according to an illustrative example, the atomic ratio of the first transition metal (M1):second transition metal (M2) can be determined, for example, in the range of 1:about 0.2-1, specifically 1:about 0.4-0.8, and more specifically 1:about 0.5-0.7.

[0053] According to an exemplary embodiment, as previously described, the catalyst may also be porous because the two transition metals are supported in monoatomic form on a porous nitrogen-doped carbon nanostructure matrix. In this regard, the monoatomic catalyst may have a specific surface area (BET) of, for example, at least about 400 m. 2 / g, specifically about 500 to 1000m 2 / g, more specifically, about 600-800m 2 The pore volume of the catalyst can be, for example, in the range of about 0.1 to 1.2 m / g. 3 / g, specifically, approximately 0.3 to 1.0 m 3 / g, more specifically, about 0.5 to 0.8 m 3 / g range. This increased porosity (or high specific surface area) allows the active sites within the catalyst to be more effectively exposed to the reactant carbon dioxide.

[0054] According to an exemplary embodiment, the specific metal surface area of ​​the single atom catalyst, as measured by nitrogen adsorption (N2 sorption), is, for example, at least about 400 m 2 / g, specifically about 500 to 1000m 2 / g, more specifically, about 600-800m 2 / g range.

[0055] Method for producing a single atom catalyst for carbon dioxide conversion According to another embodiment of the present disclosure, there is provided a method for preparing the aforementioned single atom catalyst for carbon dioxide conversion. In this regard, Figure 1 schematically illustrates a process for preparing the single atom catalyst according to an exemplary embodiment (specifically, an ion adsorption method).

[0056] Referring to the diagram, a step of forming porous carbon (or carbon nanostructures), for example, a step of pyrolyzing a carbon precursor (primary pyrolysis), can be performed. Here, carbon nanostructures can be produced by performing a single pyrolysis step without using a separate template. For example, a carbon precursor having an organic framework structure can be used. According to illustrative embodiments, the carbon precursor can be at least one selected from organic salts, polymer composites, biomass, metal-organic frameworks (MOFs), carbohydrates, etc., or a hydrate thereof. According to specific embodiments, an organic salt can be used as the carbon precursor. Since the organic salt has a carbon-carbon ring, it can be easily converted into a carbon framework when heat-treated in an inert atmosphere. Here, the cation in the organic salt can be, for example, a metal cation such as zinc, an alkali metal, or an alkaline earth metal, or an ammonium ion. According to certain embodiments, the carbon precursor may be at least one selected from, for example, sodium citrate, potassium citrate, glucose carbonate, ammonium citrate, sodium citrate, potassium citrate, calcium citrate, sodium gluconate, potassium gluconate, sodium alginate, potassium alginate, etc., or a hydrate thereof. According to certain embodiments, sodium citrate dihydrate may be used as the carbon precursor.

[0057] According to an illustrative embodiment, the carbon precursor may be converted into porous carbon by pyrolysis under conditions of an inert atmosphere and a first heat treatment temperature. Here, the inert atmosphere may be formed by at least one gas selected from argon, nitrogen, helium, etc., and more specifically, the first heat treatment may be performed in a nitrogen atmosphere. The first heat treatment temperature may be adjusted to, for example, about 400 to 1000°C, specifically about 500 to 900°C, and more specifically about 600 to 850°C. According to an illustrative embodiment, the heating rate in the first pyrolysis step may be adjusted in consideration of the degree of morphological formation of the carbon skeleton. For example, it may be in the range of about 1 to 10°C / min, specifically about 2 to 8°C / min, and more specifically about 4 to 6°C / min. Meanwhile, the heat treatment time is not particularly limited, but may be adjusted to, for example, at least about 0.1 hours, specifically about 0.2 to 5 hours, and more specifically about 0.5 to 2 hours. By such heat treatment in an inert atmosphere, a carbon precursor such as an organic salt can be converted into carbon, specifically, porous carbon.

[0058] According to illustrative embodiments, carbon produced by pyrolysis may contain residual impurities (e.g., inorganic impurities), metals (e.g., alkali metals, alkali metal oxides, alkaline earth metals, and / or alkaline earth metal oxides), etc. These residual components may clog the pore structure of the carbon framework, resulting in limited porosity. To resolve or mitigate this issue, acid washing or acid leaching (primary acid washing) may be performed. Examples of acids that can be used include at least one selected from sulfuric acid, nitric acid, phosphoric acid, hydrochloric acid, and formic acid. More specifically, sulfuric acid and / or hydrochloric acid may be used. Furthermore, acid washing may be performed in the form of an aqueous acid solution, and the concentration may be adjusted to, for example, about 0.1 to 2 M, specifically about 0.5 to 1.5 M, but this is merely illustrative.

[0059] Meanwhile, referring again to FIG. 1, after the porous carbon is prepared, nitrogen can be doped therein to form a nitrogen (N)-doped porous carbon nanostructure. As described above, nitrogen doping occurs when nitrogen is coordinated or chelated with a transition metal (specifically, a transition metal ion) to form N. x Nitrogen atoms forming active sites of the form can be provided.

[0060] In an exemplary embodiment, nitrogen doping can be performed using a nitrogen source containing nitrogen atoms in its molecule, such as a compound containing both nitrogen and carbon or a nitrogen-containing organic compound. To this end, a nitrogen source can be first contacted with the porous carbon to form a nitrogen source-porous carbon composite. The nitrogen source is not limited to a specific compound, and at least one precursor can be used, as long as it can be carbonized by heat treatment, such as pyrolysis. Examples of the nitrogen source include urea, melamine, ammonia, polyamide-imide resin, adenine, pyrrole, polypyrrole, polyvinylpyrrole, 3-methylpolypyrrole, acrylonitrile, polyacrylonitrile, phenanthroline, polyacrylonitrile-polymethacrylic acid copolymer, pyrazole, vinylpyridine, polyvinylpyridine, pyrimidine, piperazine, pyran, carbamide, morpholine, imidazole, 1-methylimidazole, 2-methylimidazole, quinoxaline, aniline, polyaniline, polyimide, benzimidazole, polybenzimidazole, polyamide, ethylenediamine, and cyanamide. More specifically, polypyrrole can be used as the nitrogen source. Polypyrrole contains one nitrogen atom per pyrrole monomer molecule, allowing for efficient nitrogen doping. Furthermore, coating porous carbon with polypyrrole allows for uniform doping across the carbon surface.

[0061] According to an illustrative example, the previously prepared porous carbon can be dispersed in a liquid medium (dispersion medium), and then a nitrogen source can be added and brought into contact with the porous carbon. Here, the liquid medium can be at least one selected from water, aliphatic alcohols having 1 to 4 carbon atoms (e.g., methanol, ethanol, n-propanol, isopropanol, and / or n-butanol), etc. Specifically, a mixed solvent of ethanol and water can be used as the liquid medium. Here, the volume ratio of ethanol to water can be adjusted, for example, within the range of about 1:0.1-2, specifically about 1:0.4-1.6, and more specifically about 1:0.8-1.2, but this can be understood as an illustrative example.

[0062] According to an exemplary embodiment, for nitrogen doping, a nitrogen source may first be coated or attached to the porous carbon in the carbon dispersion. In this regard, the amount of nitrogen source added to the carbon dispersion may be determined based on the amount of nitrogen to be incorporated (doped) into the porous carbon. However, since not all of the added nitrogen source is used for doping, an amount in excess of the actual amount may be added. Exemplarily, the amount of nitrogen source added relative to the porous carbon may be determined in the range of about 0.1 to 0.6, specifically about 0.12 to 0.4, and more specifically about 0.15 to 0.25, by weight.

[0063] Meanwhile, according to an exemplary embodiment, a nitrogen source in the form of a monomer can be added to a porous carbon dispersion and then polymerized in situ to coat or attach the nitrogen source to the porous carbon (forming a nitrogen source-porous carbon composite). For example, a monomer such as pyrrole, phenanthroline, aniline, or melamine can be added and polymerized using a chemical oxidation method using an oxidant. Here, the oxidant can be used in the form of a solution for efficient polymerization. According to an exemplary embodiment, the oxidant can be, for example, ammonium persulfate, ammonium peroxydisulfate, sodium persulfate, potassium persulfate, iron(III) chloride, iron(II) chloride, hydrogen peroxide, or iron(III)-alkylbenzenesulfonate, either alone or in combination. The amount of the oxidant used can be, for example, about 5 to 30 times, specifically about 8 to 20 times, and more specifically about 10 to 15 times the moles of the monomer. Alternatively, the polymerization reaction time can be determined within the range of, for example, about 0.1 to 30 hours, specifically about 1 to 20 hours, and more specifically about 1.5 to 10 hours.

[0064] Referring again to FIG. 1, two transition metal precursors can be introduced into the nitrogen source-porous carbon composite by ion adsorption. The transition metal precursors are typically used in the form of water-soluble compounds. For example, they can be in the form of halides, organic acid salts, inorganic acid salts, hydroxides, complexes, combinations thereof, or hydrates of the aforementioned compounds. For example, the inorganic acid salts can be at least one selected from nitrates, sulfates, carbonates, halides, etc. Furthermore, the organic acid salts can be at least one selected from acetates, alkoxides, glutarates, etc. Additionally, the halide salts can be at least one selected from fluorides, chlorides, bromides, iodides, etc.

[0065] Meanwhile, when nickel is used as the first transition metal (M1), a nickel compound (e.g., a salt or complex) having an oxidation valence of 2+ can be used. 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.

[0066] Furthermore, when manganese is used as the second transition metal (M2), a manganese compound (specifically, a salt or complex) having an oxidation state of 2+ can be used. Examples of such manganese compounds include at least one selected from manganese nitrate, manganese sulfate, manganese acetate, manganese chloride, manganese carbonate, manganese acetylacetonate complex, and the like, or a hydrate thereof. In a specific example, manganese nitrate and / or a hydrate thereof (e.g., Mn(NO3)2·6H2O) can be used as the manganese compound.

[0067] According to an illustrative embodiment, the two transition metal precursors can be provided in the form of a solution (e.g., an aqueous solution). The two precursors can be added to a single solution, or individual transition metal precursor solutions can be prepared and then combined. Exemplarily, the concentrations of the first transition metal precursor and the second transition metal precursor in the precursor solution are not particularly limited, but can be adjusted to, for example, about 0.2-0.8 mg / mL (specifically, about 0.4-0.6 mg / mL) and about 0.1-0.7 mg / mL (specifically, about 0.3-0.5 mg / mL), taking into account the degree of ion adsorption. The previously prepared nitrogen source-porous carbon composite can be added to the two transition metal precursor solutions prepared in this manner. Alternatively, a dispersion (e.g., an aqueous dispersion) of the nitrogen source-porous carbon composite can be prepared, and then the two transition metal precursors or their solutions can be added to the dispersion. According to an illustrative embodiment, the total weight ratio of the transition metal to the nitrogen source-porous carbon composite may depend on the amount of the transition metal supported in the final catalyst. Taking into account the transition metal that is not actually supported, the ratio may be adjusted to, for example, a range of 1: about 0.04 to 0.16, specifically 1: about 0.06 to 0.14, and more specifically 1: about 0.08 to 0.12, but this may be understood as an illustrative example.

[0068] Through the above process, a nitrogen source-porous carbon composite adsorbed with two transition metal precursors (ions) can be formed, followed by a pyrolysis step to form a single-atom catalyst. Here, pyrolysis (secondary pyrolysis) can be performed under conditions of an inert atmosphere and a second heat treatment temperature. As described above, the inert atmosphere can be formed by at least one gas selected from argon, nitrogen, helium, etc., and more specifically, a nitrogen atmosphere. The second heat treatment temperature can be adjusted, for example, within the range of about 700 to 1000°C, specifically about 750 to 900°C, and more specifically about 780 to 850°C. Here, during the secondary pyrolysis step, the heating rate can be adjusted taking into account the degree of metal ion aggregation. For example, the heating rate can be within the range of about 0.5 to 8°C / min, specifically about 1 to 6°C / min, and more specifically about 2 to 4°C / min.

[0069] The above-mentioned secondary pyrolysis step forms a product in which two transition metals (M1 and M2) are supported on the nitrogen-doped carbon nanostructure. Here, the single atoms of the two transition metals are bonded or coordinated with nitrogen (i.e., M1-N x Moiety and M2-N x The carbon nanostructures can be doped with or incorporated with the moiety.

[0070] What is noteworthy about the second pyrolysis stage is that the bond (or connection) between the two transition metal atoms can be changed by adjusting the heat treatment time. Specifically, in the early stage of pyrolysis, the adsorbed first and second transition metals are in the form of M1-N x Moiety and M2-N x Each of the moieties M1-N x -N x Then, as the heat treatment time passes, M1 and M2 change to a directly connected form, forming N x -M1-M2-N xTherefore, in this embodiment, due to the indirect bonding between the two transition metals, the secondary pyrolysis reaction time can be adjusted to, for example, about 8 hours or less, specifically about 6 hours or less, more specifically about 0.5 to 4 hours, and particularly specifically about 0.8 to 3 hours. According to a specific embodiment, the secondary pyrolysis reaction time can be adjusted to within about 2 hours, specifically about 1.5 hours. As the pyrolysis temperature increases, the transfer between the metal ions (M1, M2) is promoted to form N. x -M1-M2-N x This induces the binding of

[0071] According to an illustrative embodiment, after the second heat treatment, acid washing or acid leaching can be selectively performed to remove transition metals that are not bonded to nitrogen atoms and transition metal particles that have aggregated at high temperatures, and to distribute monoatoms in the nitrogen-doped carbon nanostructures (secondary acid washing). Here, the type and / or concentration of the acid used in the first acid washing step can be adjusted to be the same or different within the range. In this manner, a monoatomic catalyst can be obtained by performing pyrolysis or pyrolysis and acid washing, followed by at least one of the following conventional post-treatments, such as water washing and drying. Such post-treatment processes are widely known in the art, and therefore will not be described in detail here.

[0072] Meanwhile, in the single-atom catalyst according to this embodiment, two transition metals bonded to nitrogen in a nitrogen-doped carbon nanostructure are adjacent to each other and indirectly bonded to each other, which has a significant effect on the electronic structure, thereby controlling the degree of adsorption with carbon dioxide conversion intermediates, and as a result, low overpotential, high current density, and high carbon monoxide selectivity can be achieved.

[0073] In this regard, the carbon dioxide uptake capacity (CO2 uptake capacity; 25°C) of the single atom catalyst can be, for example, at least about 1 mmol / g, specifically in the range of about 1.5 to 3 mmol / g, and more specifically in the range of about 2 to 2.5 mmol / g.

[0074] Electrodes (catalytic electrodes) and systems for carbon dioxide reduction reactions According to another embodiment of the present disclosure, two types of transition metal single-atom catalysts can be loaded onto an electrochemical electrode and used to implement a carbon dioxide conversion system (specifically, a system that selectively reduces carbon dioxide to carbon monoxide). In particular, the catalyst-loaded electrode can be used as a cathode in a carbon dioxide conversion system where carbon monoxide is produced.

[0075] According to an illustrative embodiment, when manufacturing an electrode, a catalyst may be loaded onto a conductive substrate. However, the loading method is not particularly limited, and methods known in the art, such as loading using a slurry, deposition, spray coating, etc., may be applied.

[0076] For example, in the case of the slurry loading method, a slurry of the previously produced catalyst (the dispersion medium can 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 produced. Here, the concentration of the catalyst slurry can be adjusted to, for example, about 5 to 30 wt %, specifically, about 10 to 20 wt %. Next, the slurry can be applied to an electrode substrate (e.g., a conductive substrate) and then dried to produce a catalyst electrode.

[0077] Alternatively, a deposition method may be used, in which case the catalyst layer to be loaded may be adjusted by adjusting the deposition rate, drying temperature, etc., and cleaning may be performed after deposition.

[0078] According to illustrative embodiments, the substrate on which the carbon dioxide conversion catalyst is loaded may have various shapes, such as a plate, rod, mesh, disk, wire, paper, etc. The conductive substrate may be made of a material that maintains conductivity even when exposed to an oxidizing atmosphere. For example, the conductive substrate may be made of at least one material (including alloys) selected from valve metals (e.g., titanium, aluminum, chromium, etc.), stainless steel, etc., or carbon.

[0079] In an exemplary embodiment, the amount of catalyst loaded is, for example, about 0.2-5 mg / cm based on the electrode. 2 , specifically about 0.5 to 3 mg / cm 2 , more specifically about 0.8 to 2 mg / cm 2 , but this can be understood as an exemplary purpose.

[0080] According to one embodiment of the present disclosure, a carbon dioxide conversion system can be realized in which carbon dioxide is reduced using the electrochemical electrode as a cathode to generate 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 and react with carbon dioxide at the cathode to generate carbon monoxide gas and hydroxide ions (OH - The carbon monoxide gas produced here is discharged to the outside, and the hydroxide ions move to the anode. The hydroxide ions that move to the anode lose electrons (are oxidized) and are converted into oxygen gas, which can then be discharged.

[0081] 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.). In addition, in the case of a three-electrode system, a reference electrode may also be included. 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.

[0082] According to an exemplary embodiment, the carbon dioxide conversion system uses, as the cathode, one of a pair of electrodes, an electrochemical electrode containing or having the above-described catalyst attached thereto, and the remaining electrode, the anode, can be of a type known in the art. In this regard, the anode can also be made of a conductive material. For example, a conductive material applicable to the cathode can be selected from materials that exhibit particularly low overvoltage while easily generating carbon dioxide and not decomposing upon contact with an electrolyte (e.g., an aqueous electrolyte). For example, the conductive material can be one selected from nickel, carbon, titanium, stainless steel, etc. (including alloys). In an electrochemical carbon dioxide conversion system according to a specific embodiment, the cathode and anode can each be operated in full contact with or immersed in an electrolyte-containing aqueous solution.

[0083] Meanwhile, according to an illustrative embodiment, an aqueous electrolyte solution (which may be alkaline) in which an electrolyte is dissolved in an aqueous medium can be supplied to the carbon dioxide conversion system. Here, the alkaline medium (or electrolyte) can be at least one selected from potassium hydroxide, potassium bicarbonate, potassium bicarbonate, sodium bicarbonate, sodium hydroxide, lithium hydroxide, etc. Exemplarily, the concentration of the alkaline medium can be, for example, about 0.05 to 3 M, specifically about 0.1 to 2 M, more specifically about 0.5 to 1.5 M. Furthermore, the pH of the alkaline medium (specifically, the electrolyte-containing aqueous solution) can be adjusted to a basic range, for example, about 9 to 14, specifically about 11 to 14, more specifically about 12 to 14.

[0084] Alternatively, a water splitting 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 can 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 perfluorinated sulfonic acid polymer materials such as Nafion, a trade name), and the like.

[0085] Generally, under thermodynamic standard conditions, the reduction reaction of carbon dioxide can occur 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 potential at which a current of the same intensity flows between such a pair of opposing electrodes is different at each electrode, and the voltage at this time corresponds to the overpotential at each electrode.

[0086] 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 at the cathode, it can operate at a low overpotential. In addition, the carbon dioxide reduction reaction can be carried out at a temperature of, for example, about 15 to 40°C, specifically about 20 to 30°C, and more specifically, room temperature.

[0087] When an electrochemical electrode loaded with a single-atom catalyst according to this embodiment is used as a cathode, a current of at least about 200 mA / cm is obtained for the carbon dioxide reduction (carbon monoxide production) reaction in 1 M KOH (pH 14) at an overpotential of 0.297 V (vs. RHE). 2 , specifically about 220 to 400 mA / cm 2 , more specifically, about 250 to 350 mA / cm 2 , more specifically, about 280 to 320 mA / cm 2 Alternatively, the Tafel slope can be, for example, about 200 mV / dec or less, specifically about 180 mV / dec or less, and more specifically about 160 mV / dec or less.

[0088] Meanwhile, the Faraday efficiency is an index for evaluating the efficiency and selectivity of an electrochemical reaction, and can be expressed by the following mathematical formula 1.

[0089] [Mathematical formula 1]

number

[0090] In the above equation, 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.

[0091] According to this embodiment, the faradaic efficiency of the electrochemical reaction for converting carbon dioxide to carbon monoxide can be at least about 90%, specifically at least about 93%, and more specifically at least about 95%.

[0092] Preferred examples are presented below 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.

[0093] Example Details of the materials used in this example, catalyst analysis and electrochemical performance tests are as follows:

[0094] A. Substance Pyrrole (C4H5N), ammonium persulfate ((NH4)2S2O8), sodium citrate dihydrate (C6H5Na3O7·2H2O), sulfuric acid, hydrochloric acid, potassium hydroxide, ethanol, nickel nitrate hexahydrate, and nickel phthalocyanine (C 32 H 16 N, N, and Ni) were purchased from Sigma-Aldrich.

[0095] Manganese nitrate hexahydrate (Mn(NO3)2·6H2O) was purchased from Alfa Aesar.

[0096] Sigracet 39BB carbon paper and commercial Ag / C electrodes were purchased from Dioxide Materials.

[0097] Deionized water (DI, 18 MΩcm) was purchased from Millipore and used throughout the entire experimental process.

[0098] B.Physicochemical catalyst analysis Scanning electron microscope (SEM) analysis was performed using JSM-7401F.

[0099] High-resolution transmission electron microscopy (HR-TEM) and elemental mapping analysis were performed using a JEM-2100F equipped with a HAADF-STEM detector.

[0100] The metal crystal planes were confirmed using X-ray diffraction (XRD; PANalytical) analysis, where measurements were taken using Cu Kα radiation at 40 kV and 100 mA over a range of 10–80° at a scan rate of 6° per minute with an interval of 0.01°.

[0101] Using X-ray absorption fine structure (XAFS; PLS-II) analysis, we obtained the Ni K-edge XAFS spectrum in the photon energy range of 8132-9078 eV and the Mn K-edge XAFS spectrum in the photon energy range of 6339-7284 eV.

[0102] The metal content (wt%) of the prepared catalyst was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES, NexION 300X).

[0103] High-performance X-ray photoelectron spectroscopy (XPS; VG multilab 2000) was used to obtain XPS spectra of surface elements.

[0104] Ni K-edge and Fe K-edge XANES (X-ray absorption near edge structure) spectra were acquired from the Pohang Accelerator Laboratory (PLS-II, 7D beamline), and a portion of the Demeter program was used for XAS data analysis.

[0105] The N2 absorption isotherm was determined at -196°C by Brunauer-Emmett-Teller (BET) analysis (Quadrasorb SI), 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 After coating each of the catalysts with a loading amount of 1000 ppm, the working electrode was set. Hg / HgO (sat. 1M NaOH), CoFeMnO were used as the reference electrode, counter electrode, and ion exchange membrane. x Foil and Sustainion anion exchange membranes (Dioxide Materials) were used, respectively, and all measurements were expressed vs. RHE (reversible hydrogen electrode).

[0107] The electrolyte was a 1 M KOH solution (pH: 14), which was circulated at a rate of 1.5 mL / min to the working electrode, and the same electrolyte was used for the counter electrode. Carbon dioxide gas was continuously supplied to the carbon paper gas diffusion layer in the cathode at a flow rate of 20 mL / min.

[0108] Electrochemical carbon dioxide conversion performance was measured using chronopotentiometry at -40mA / cm 2 , -70mA / cm 2 , -120mA / cm 2 , -200mA / cm 2 , -300mA / cm 2 , -350mA / cm 2 , and -370mA / cm 2The reaction was carried out for 1 hour under the above conditions, and the generated gas was analyzed by gas chromatography (Agilent 7890).

[0109] Catalyst synthesis In the examples and comparative examples, the catalysts were prepared according to the procedure shown in Figure 1, i.e., dual monatomic (Ni and Mn) catalysts (non-bonding NiMn / NPC and direct bonding NiMn / NPC) and single metal monatomic metal catalysts (Ni / NPC and Mn / NPC), respectively.

[0110] Example 1 Preparation of indirectly bonded NiMn / NPC catalysts Sodium citrate dihydrate was pyrolyzed in a N2 atmosphere at 800°C for 1 hour (primary pyrolysis), and the resulting powder was washed with 1M H2SO4 solution (Sigma) to remove residual inorganic impurities. The resulting porous carbon was dispersed in a mixture of 40 mL of ethanol and 40 mL of distilled water using ultrasonic treatment, followed by the addition of 2.2 mmol of pyrrole monomer and stirring for 30 minutes. A solution of 26 mmol of ammonium persulfate dissolved in 10 mL of distilled water as an oxidant was then added dropwise to the dispersion while stirring for 2 hours to synthesize a polypyrrole-porous carbon composite (PPyPC) in which polypyrrole was coated on the porous carbon.

[0111] To prepare the Ni-Mn binary single-atom catalyst, an ion adsorption process was utilized. First, the polypyrrole-porous carbon composite was added to 40 mL of a metal nitrate precursor solution (0.5 mg / mL nickel nitrate and 0.4 mg / mL manganese nitrate) and stirred for one day to allow the metal ions to be adsorbed onto the polypyrrole-porous carbon composite (Ni 2+ -Mn 2+ -PPyPC). This was centrifuged, collected, dried, and then pyrolyzed in a N2 atmosphere at 800°C for 1 hour (secondary pyrolysis). Finally, it was washed with 1M HCl solution to remove the metal nanoparticles that had aggregated at high temperature, producing the NiMn / NPC catalyst.

[0112] Comparative Example 1 Preparation of direct-bonded NiMn / NPC catalysts The NiMn / NPC catalyst was prepared in the same manner as in Example 1, except that the secondary pyrolysis time was adjusted to 4 hours.

[0113] Comparative Example 2 Preparation of Ni / NPC catalyst A Ni / NPC catalyst was prepared in the same manner as in Example 1, except that only nickel nitrate was used as the metal precursor.

[0114] Comparative Example 3 Preparation of Mn / NPC catalyst A Mn / NPC catalyst was prepared in the same manner as in Example 1, except that only manganese nitrate was used as the metal precursor.

[0115] Comparative Example 4 A commercial Ag / C catalyst was prepared.

[0116] Catalyst characterization The SEM, TEM, HAADF-STEM / element mapping images, intensity profiles, and corresponding EEL spectra of the NiMn / NPC catalyst are shown in Figures 3a–3e. As shown in Figures 3a and 3b, the NiMn / NPC catalyst possesses a well-developed hierarchical framework structure with open pores in multidirectional channels. The HAADF-STEM analysis shown in Figure 3c reveals that Ni and Mn atoms are uniformly distributed on the porous carbon, and the element mapping analysis confirms that Ni and Mn atoms are uniformly distributed throughout the carbon structure. As seen in Figure 3d, numerous metal pairs are adjacent with an average distance of approximately 4.0 Å. This suggests the formation of monoatomic pairs within the NiMn / NPC catalyst. Furthermore, the EEL spectrum analysis shown in Figure 3e confirms that each monoatomic pair contains one Ni atom and one Mn atom. This indicates the formation of adjacent Ni-Mn monoatomic active sites within the NiMn / NPC catalyst.

[0117] The XRD patterns of the binary monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC) and the single metal monatomic metal catalysts (Ni / NPC and Mn / NPC), respectively, are shown in Figure 4 .

[0118] Referring to the figure, in all of the catalysts prepared in Example 1 and Comparative Examples 1 to 3, no peaks related to other metals were observed other than the carbon peak (002 plane) around 22°C. This result indicates that the metal is uniformly distributed on the carbon support in a single atomic state without forming metal crystalline particles.

[0119] ICP analysis was carried out on the metals (Ni and / or Mn) in the catalysts prepared in Example 1 and Comparative Examples 1 to 3, and the results are shown in Table 1 below.

[0120] [Table 1]

[0121] The BET analysis results for the binary monatomic (Ni and Mn) catalyst (indirectly bonded NiMn / NPC) and the single metal monatomic metal catalysts (Ni / NPC and Mn / NPC), respectively, are shown in Figure 5 .

[0122] Referring to the figure, the catalysts prepared in Example 1 and Comparative Examples 2 and 3 have similar pore structures, specific surface areas (651 to 662 m), and 2 The reason for the similar pore structure is that the heat treatment (secondary pyrolysis) was carried out at the same temperature, and it is believed that the increased porous structure (high specific surface area and pore volume) allows reactants to access more active sites, providing efficient transport channels for electrons and carbon dioxide reduction reaction (CO2RR) species.

[0123] The XANES analysis results for Ni phthalocyanine, Ni foil, bicomponent monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), and monometallic monatomic metal catalyst (Ni / NPC), respectively, and the XANES analysis results for Mn phthalocyanine, Mn foil, bicomponent monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), and monometallic monatomic metal catalyst (Mn / NPC), respectively, are shown in Figure 6a and Figure 6b.

[0124] Referring to the figure, a negative shift was observed from the pre-edge position of the Ni K-edge spectrum for the indirectly bound NiMn / NPC catalyst and the directly bound NiMn / NPC catalyst, respectively, relative to the Ni / NPC catalyst, while a positive shift was observed in the pre-edge position of the Mn K-edge spectrum relative to the Mn / NPC catalyst.

[0125] Considering the above results, we can conclude that in the two NiMn / NPC catalysts, the Ni and Mn atoms electrically interact with each other through indirect or direct bonding. Furthermore, by comparing the degree of shift, we confirmed that the electronic structure of Ni, which acts as the main site for CO activity, has a relatively lower oxidation state in the indirectly bonded NiMn / NPC catalyst compared to the directly bonded NiMn / NPC catalyst.

[0126] The EXAFS analysis results for Ni phthalocyanine, Ni foil, bicomponent monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), and monometallic monatomic metal catalyst (Ni / NPC), respectively, and the EXAFS analysis results for Mn phthalocyanine, Mn foil, bicomponent monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), and monometallic monatomic metal catalyst (Mn / NPC), respectively, are shown in Figure 7a and Figure 7b.

[0127] Referring to the figure, the Ni K-edge Fourier-transformed (FT) k3-weighted EXAFS spectra of the non-bonded NiMn / NPC catalyst, the direct-bonded NiMn / NPC catalyst, and the Ni / NPC catalyst all exhibited Ni-N peaks, indicating that Ni exists in a monoatomic form bonded to N. However, in the case of the direct-bonded NiMn / NPC catalyst, an additional Ni-metal peak was observed, confirming that Ni and Mn are directly bonded to each other.

[0128] Similarly, in the Mn FT-EXAFS spectra, the non-bonding NiMn / NPC catalyst, the direct bonding NiMn / NPC catalyst, and the Mn / NPC catalyst all existed in a monoatomic form, with Mn bonded to N. On the other hand, in the case of the direct bonding NiMn / NPC catalyst, a Mn-metal peak was observed, indicating that Ni and Mn existed in a directly bonded form.

[0129] 6 and 7, respectively, show that in the NiMn / NPC catalyst according to the embodiment, Ni and Mn are indirectly (or indirectly) bonded to each other and electrically interact, whereas in the NiMn / NPC catalyst according to Comparative Example 1, Ni and Mn are directly bonded to each other and electrically interact. Thus, it is believed that the bonding structure between different monoatomic metals causes differences in the degree of deformation of the electronic structure of the Ni-N site, which is the main active site for CO formation, thereby affecting the carbon dioxide conversion reaction performance.

[0130] Electrochemical performance evaluation The faradaic efficiencies for carbon monoxide production of the binary monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), the monometallic monatomic metal catalysts (Ni / NPC and Mn / NPC), and the commercial Ag / C catalyst were measured and shown in Figure 8 a.

[0131] According to the figure, in the case of the indirectly bonded NiMn / NPC catalyst according to the embodiment, the faradaic efficiency (Fe CO ), and showed a faradaic efficiency of over 95% in the range of -0.257 to -0.464 V (vs. RHE). Compared with the catalysts of Comparative Example 1 (directly bonded NiMn / NPC catalyst), Comparative Example 2 (Ni / NPC catalyst), Comparative Example 3 (Mn / NPC catalyst), and Comparative Example 4 (commercial Ag / C catalyst), it showed the highest faradaic efficiency for CO production at low voltages.

[0132] In this regard, the Ni / NPC catalyst exhibited a slow reaction rate in the formation of the *COOH intermediate in the carbon dioxide reduction reaction (problems with carbon dioxide activation in reaction 3), and the Mn / NPC catalyst exhibited the lowest faradaic efficiency, which is thought to be due to the low carbon dioxide reduction activity of a single Mn atom. Furthermore, the indirectly bonded NiMn / NPC catalyst exhibited better CO selectivity than the commercial Ag / C catalyst.

[0133] Furthermore, it is noteworthy that the indirectly bonded NiMn / NPC catalyst exhibits improved catalytic performance compared to the directly bonded NiMn / NPC catalyst, despite both containing Ni and Mn, which is believed to be due to the different electronic structures of Ni and Mn due to the bonding mode or morphology.

[0134] The current densities for carbon monoxide production of the binary monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC), the monometallic monatomic metal catalysts (Ni / NPC and Mn / NPC), and the commercial Ag / C catalyst are shown in Figure 8 b.

[0135] Referring to the figure, the indirectly bonded NiMn / NPC catalyst exhibited the lowest applied voltage at a current density of -200 mA / cm for the same amount of CO production. 2At a current density of 0.01, the applied voltage increased in the following order: indirectly coupled NiMn / NPC catalyst (-0.34 V), directly coupled NiMn / NPC catalyst (-0.36 V), Ni / NPC catalyst (-0.40 V), and commercial Ag / C catalyst (-0.66 V). This confirms that the indirectly coupled NiMn / NPC catalyst has a high CO production rate even at low voltages.

[0136] Considering the above-mentioned catalyst characterization results, the indirectly bonded NiMn / NPC catalyst has a structure in which Ni and Mn exist simultaneously as single atoms but are indirectly connected by bonds with N atoms, which is structurally distinct from the directly bonded NiMn / NPC catalyst in which Ni and Mn are directly connected to each other. As a result, it is believed that the indirectly bonded NiMn / NPC catalyst promoted the carbon dioxide conversion reaction by effectively controlling the electronic structure through electrical interactions.

[0137] The Tafel plots for carbon monoxide production for the binary monatomic (Ni and Mn) catalysts (indirectly bonded NiMn / NPC and directly bonded NiMn / NPC) and the commercial Ag / C catalyst are shown in FIG.

[0138] As shown in the figure, the slope of the Tafel plot for the indirectly bonded NiMn / NPC catalyst was significantly lower than that of the directly bonded NiMn / NPC catalyst and commercial Ag / C. These results indicate that when Ni and Mn atoms are indirectly bonded to each other, a more efficient catalytic reaction pathway is formed, promoting the carbon dioxide conversion reaction.

[0139] Simple variations and modifications of the present invention can be easily made by those skilled in the art, and all such variations and modifications can be considered to be within the scope of the present invention.

Claims

1. nitrogen-doped carbon nanostructures; The nitrogen-doped carbon nanostructure is supported on the first transition metal (M １ ) and second transition metals (M ２ )and, An electrochemical catalyst for a carbon dioxide reduction reaction, comprising: The catalyst is a first transition metal (M １ ) and second transition metals (M ２ ) are bonded to nitrogen atoms of the nitrogen-doped carbon nanostructures, respectively. １ -N ｘ Moiety and M ２ -N ｘ Moiety (N ｘ means a nitrogen atom having coordinating ability; x is in the range of 0.5 to 2.5; M １ and M ２ is indirectly bound, In the catalyst, the first transition metal (M 1 ) and the second transition metal (M 2 ) are connected to each other by an M 1 -N x moiety and an M 2 -N x moiety in the form of M 1 -N x -N x -M 2 ; The first transition metal (M 1 ) is nickel (Ni); The second transition metal (M 2 ) is manganese (Mn); The contents of the first transition metal (M 1 ) and the second transition metal (M 2 ) are determined to be in the ranges of 0.1 to 1 wt % and 0.05 to 0.8 wt %, respectively; The catalyst, wherein the atomic ratio of the first transition metal (M 1 ):second transition metal (M 2 ) is determined in the range of 1:0.2-1.

2. The catalyst according to claim 1, wherein the nitrogen-doped carbon nanostructure has porosity.

3. The catalyst is at least 400 m ２ Catalyst according to claim 2, characterized in that it has a specific surface area (BET) of 1 / g.

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

5. a) pyrolyzing a carbon precursor in an inert atmosphere at a first heat treatment temperature to prepare porous carbon; b) contacting the porous carbon with a nitrogen source to form a nitrogen source-porous carbon composite; c) adding a first transition metal (M １ ) precursors and second transition metals (M ２ ) a precursor of d) the first transition metal (M １ ) and second transition metals (M ２ The nitrogen source-porous carbon composite with the adsorbed first transition metal (M) is thermally decomposed in an inert atmosphere at a second heat treatment temperature, thereby doping the nitrogen-doped carbon nanostructure with the first transition metal (M) in the form of a single atom. １ ) and second transition metals (M ２ ) and 2. A method for producing the electrochemical catalyst for carbon dioxide reduction reaction according to claim 1, comprising:

6. 6. The method of claim 5, wherein in step a), the first heat treatment temperature is determined in the range of 400 to 1000°C, and the pyrolysis time is determined in the range of at least 0.1 hours.

7. 6. The method for producing an electrochemical catalyst according to claim 5, wherein the nitrogen source is at least one selected from the group consisting of urea, melamine, ammonia, polyamideimide resin, adenine, pyrrole, polypyrrole, polyvinylpyrrole, 3-methylpolypyrrole, acrylonitrile, polyacrylonitrile, phenanthroline, polyacrylonitrile-polymethacrylic acid copolymer, pyrazole, vinylpyridine, polyvinylpyridine, pyrimidine, piperazine, pyran, carbamide, morpholine, imidazole, 1-methylimidazole, 2-methylimidazole, quinoxaline, aniline, polyaniline, polyimide, benzimidazole, polybenzimidazole, polyamide, ethylenediamine, and cyanamide.

8. First transition metal (M １ ) precursors and second transition metals (M ２ 6. The method of claim 5, wherein each of the precursors of (a) and (b) is a halide, an organic acid salt, an inorganic acid salt, a hydroxide, a complex, or a combination thereof.

9. 6. The method of claim 5, wherein in step d), the second heat treatment temperature is determined to be in the range of 700 to 1000° C., and the pyrolysis time is determined to be 8 hours or less.

10. 6. The method for producing an electrochemical catalyst according to claim 5, wherein the carbon precursor is at least one selected from the group consisting of sodium citrate, potassium citrate, glucose carbonate, ammonium citrate, sodium citrate, potassium citrate, calcium citrate, glucose carbonate, sodium gluconate, potassium gluconate, sodium alginate, and potassium alginate, or a hydrate thereof.

11. 6. The method for preparing an electrochemical catalyst according to claim 5, further comprising the step of performing an acid washing step after performing step a) and / or step d), wherein the acid is at least one selected from the group consisting of sulfuric acid, nitric acid, phosphoric acid, hydrochloric acid, and formic acid.

12. an electrode substrate; a single-atom metal catalyst loaded on the electrode substrate; An electrode for a carbon dioxide reduction reaction comprising:

10. The electrode according to claim 1, wherein the monoatomic metal catalyst is an electrochemical catalyst for the carbon dioxide reduction reaction.

13. The loading of the bimetallic monoatomic catalyst in the electrode is 0.2-5 mg / cm ２ 13. The electrode according to claim 12, wherein the thickness is in the range of

14. The electrode has a current density of at least 200 mA / cm for the carbon dioxide reduction reaction in 1 M KOH (pH 14) and at an overpotential of 0.297 V (vs. RHE). ２ 13. The electrode of claim 12, characterized in that it exhibits a current density of 0.1 V / dec or less and a Tafel slope of 200 mV / dec or less.

15. an anode and a cathode as electrochemical electrodes electrically connected to an external power source; an aqueous electrolyte filled 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 on the anode side, while carbon monoxide is generated on the cathode side; the cathode comprises a monoatomic metal catalyst loaded on an electrode substrate; The system of claim 1 , wherein the monoatomic metal catalyst is an electrochemical catalyst for the carbon dioxide reduction reaction.

Images