Single-atom catalyst and method of preparing the same

Abstract

To provide a single-atom catalyst.SOLUTION: This single-atom catalyst includes a nitrogen-doped carbon structure and a single-atom metal, wherein the single-atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure. When using an electrochemical cell having a pair of electrodes including the single-atom catalyst in an electrochemical advanced oxidation process for water treatment, organic contaminants in wastewater are efficiently degradable under mild pH conditions.SELECTED DRAWING: Figure 1a

Description

Technical Field

[0001] The proposed technology relates to single-atom catalysts and methods for manufacturing the same.

Background Art

[0002] Electrochemical advanced oxidation processes (EAOPs) have high efficiency, the use of renewable energy, and the potential for modularization of distributed systems, which has increased interest in electrochemical advanced oxidation processes in the field of water treatment.

[0003] Electrochemical advanced oxidation processes involve the generation of hydroxyl radicals (·OH) at the anode and the oxidation of refractory organic pollutants in wastewater using these radicals. The cathode generates hydrogen peroxide (H 2 O 2 ), which is a precursor of hydroxyl radicals, by the selective reduction of oxygen.

[0004] In the past decade, electrochemical advanced oxidation processes have developed significantly, and various studies have continued to improve them to date. Recently, research has also been conducted on the use of single-atom catalysts in electrochemical advanced oxidation processes.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] According to one aspect of the present disclosure, an object is to provide a single-atom catalyst that can expand the optimal pH range of electrochemical advanced oxidation and prevent the adsorption of hydroxyl radicals to the electrode.

Means for Solving the Problems

[0007] According to the present disclosure, a single-atom catalyst is provided, which includes a nitrogen-doped carbon structure and a single-atom metal, and the single-atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure.

[0008] According to one embodiment, the carbon structure can be carbon black.

[0009] According to one embodiment, the single-atom metal can be cobalt (Co).

[0010] According to one embodiment, the content of the single-atom metal in the single-atom catalyst can be more than 0 wt% and less than 3 wt%.

[0011] According to the present disclosure, a method for manufacturing a single-atom catalyst is provided, which includes steps of mixing a single-atom metal precursor and a nitrogen dopant precursor in a solvent to produce a mixed solution, mixing carbon structure powder into the mixed solution to produce a suspension, shaking and drying the suspension to produce a single-atom catalyst precursor, and firing the single-atom catalyst precursor to produce a single-atom catalyst.

[0012] According to the present disclosure, an electrode is provided, which includes a substrate and a catalyst layer coated on the substrate, and the catalyst layer includes the aforementioned single-atom catalyst.

[0013] According to one embodiment, the substrate can include stainless steel.

[0014] According to one embodiment, the ratio of the thickness of the substrate to the thickness of the catalyst layer can be 10:1 to 1000:1.

[0015] According to the present disclosure, an electrochemical cell including the aforementioned electrode as a cathode and an anode is provided.

[0016] According to the present disclosure, an electrochemical water treatment method using the aforementioned electrochemical cell is provided.

Advantages of the Invention

[0017] According to one embodiment of the present disclosure, the electrochemical advanced oxidation process can be carried out under various pH conditions, and the efficiency of removing organic substances in wastewater by the electrochemical advanced oxidation process can be improved.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, the present disclosure will be described in detail with reference to the accompanying drawings. However, this is merely exemplary, and the present disclosure is not limited to the specific embodiments described by way of example.

[0020] In the present disclosure, the "single-atom catalyst" refers to a catalyst in which a metal is dispersed on a support in a single-atom size.

[0021] A single-atom catalyst according to one aspect of the present disclosure includes a nitrogen-doped carbon structure and a single-atom metal, and the single-atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure.

[0022] The nitrogen-doped carbon structure serves as a support on which the metal of the single-atom catalyst is dispersed. The nitrogen atoms contained in the carbon structure form a coordination bond with the metal, so that the metal is fixed in the carbon structure in the form of single atoms. Also, when a metal oxide is used as a support, hydroxyl radicals (·OH) generated in the electrochemical advanced oxidation process are adsorbed on the surface of the metal oxide, which can reduce the water treatment efficiency by electrochemical advanced oxidation. In contrast, the carbon structure can have the advantage of not adsorbing hydroxyl radicals. The nitrogen doping method of the nitrogen-doped carbon structure and the type of carbon structure are not particularly limited as long as the metal can be dispersed in the form of single atoms.

[0023] In one embodiment, the carbon structure can be carbon black. As the carbon structure, graphene, graphene oxide, carbon nanotubes (CNT), graphite, etc. can be used. From the viewpoints of cost and ease of mass production, the carbon structure is preferably carbon black.

[0024] The single-atom metal of the single-atom catalyst corresponds to the reaction point of the catalyst. The single-atom metal forms a coordination bond with the nitrogen atoms of the nitrogen-doped carbon structure, and is thus fixed in the nitrogen-doped carbon structure in the form of single atoms. The single-atom metal has the advantage of providing a wider reaction active site compared with the case where metal atoms are aggregated. As the single-atom metal of the single-atom catalyst of the present disclosure, transition metals such as Co, Zn, V, Cr, Fe, Ni, Cu, Zr, Nb, Mo, etc. can usually be used, but it is not limited thereto.

[0025] In one embodiment, the single-atom metal can be cobalt (Co). Among the above-mentioned transition metals, cobalt (Co) is advantageous because it has high activity for both the generation of hydroxyl radicals through the hydroxide reaction at the anode and the generation of hydrogen peroxide through the selective oxygen reduction reaction at the cathode when the single-atom catalyst is used for electrochemical advanced oxidation.

[0026] In one embodiment, the content of the single-atom metal in the single-atom catalyst can be more than 0 wt% and less than 3 wt%. When the content of the single-atom metal in the single-atom catalyst is 3 wt% or more, the excessive single-atom metal can form a metal oxide, and the distance between adjacent single-atom metals in the process of manufacturing the single-atom catalyst such as heat treatment is insufficient and they aggregate with each other to form clusters or nanoparticles. In one embodiment, the content of the single-atom metal in the single-atom catalyst can specifically be more than 0 wt% and less than or equal to 2 wt%, and more specifically can be 0.5 wt% or more and 1 wt% or less.

[0027] According to another aspect of the present disclosure, a method for manufacturing a single-atom catalyst is provided, and this method includes steps of mixing a single-atom metal precursor and a nitrogen dopant precursor in a solvent to produce a mixed solution, mixing carbon structure powder into the mixed solution to produce a suspension, shaking and drying the suspension to produce a single-atom catalyst precursor, and firing the single-atom catalyst precursor to produce a single-atom catalyst.

[0028] The solvent in the step of manufacturing the mixed solution must have polarity so that the single-atom metal precursor and the nitrogen dopant precursor can be dissolved, and must be easily volatile in the drying step. From this perspective, the solvent is preferably alcohol, and from the perspective of reducing manufacturing costs, ethanol is preferably used.

[0029] The single-atom metal precursor may be a substance containing the single-atom metal of the single-atom catalyst. For example, when the single-atom metal is cobalt (Co), the single-atom metal precursor can be cobalt chloride hydrate.

[0030] The nitrogen dopant precursor contains a substance that provides nitrogen which is a dopant of the nitrogen-doped carbon structure of the single-atom catalyst.

[0031] In the above-mentioned solvent, the single-atom metal precursor and the nitrogen dopant precursor are mixed at a predetermined temperature in a predetermined molar ratio to form a mixed solution.

[0032] After that, in the suspension manufacturing step, carbon structure powder is introduced into the mixed solution at a predetermined molar ratio with respect to the nitrogen dopant precursor to obtain a suspension. For example, the carbon structure powder can be introduced at a weight ratio of about 8:1 to 10:1 with respect to the dopant precursor.

[0033] In the step of manufacturing the single-atom catalyst precursor, the suspension is shaken for 8 to 12 hours to make the carbon structure powder uniformly dispersed in the suspension. Then, the suspension is dried at a temperature of about 80 to 120 °C to volatilize the solvent, thereby obtaining a single-atom catalyst precursor.

[0034] In the single-atom catalyst manufacturing step, the single-atom catalyst precursor is heat-treated in an inert gas atmosphere for about 1 to 5 hours. The firing temperature can be maintained constant, or can be increased from a low temperature to a predetermined heating rate. For example, the firing temperature can be in the temperature range of about 500 to 700 °C, and the above firing temperature can be reached at a heating rate of about 5 °C / min to about 10 °C / min from room temperature. In one embodiment, the firing temperature can specifically be about 550 to 700 °C, more specifically about 600 to 650 °C. Through the above-mentioned firing, nitrogen atoms of the nitrogen dopant precursor are doped into the carbon structure. Also, the nitrogen atoms of the nitrogen dopant precursor and the metal atoms of the single-atom metal precursor form a coordination bond.

[0035] As a result, a single-atom catalyst in which metal atoms are arranged on the carbon structure in the form of single atoms is obtained, and the obtained single-atom catalyst can be the single-atom catalyst mentioned in one aspect of the present disclosure.

[0036] According to another aspect of the present disclosure, an electrode is provided, which includes a substrate and a catalyst layer coated on the substrate, and the catalyst layer includes the single-atom catalyst described above.

[0037] The substrate serves as a support for the catalyst layer, is directly connected to a power source, and transmits the current applied during power supply to the catalyst layer. To fulfill the above-described role, the substrate must have high adhesion to the catalyst layer and high electrical conductivity. Furthermore, it must have oxidation stability so that it can maintain electrical conductivity during the electrochemical high oxidation process. There is no limitation on the substance used as the substrate as long as it has the above-described characteristics.

[0038] In one embodiment, the substrate can include stainless steel. Stainless steel corresponds to a substance having the above-described adhesion to the catalyst layer and high electrical conductivity.

[0039] The catalyst layer includes the above-described single-atom catalyst and is coated on the substrate. There is no limitation on the method of coating the catalyst layer. The single-atom metal sites of the catalyst layer act as reaction sites, and when an electric current is applied through the substrate, oxidation or reduction reactions are carried out according to the polarity.

[0040] In one embodiment, the ratio of the thickness of the substrate to the thickness of the catalyst layer can be 10:1 to 1000:1. If the thickness ratio is less than 10:1 or exceeds 1000:1, there is a possibility that the electrical energy applied to the catalyst layer through the substrate will decrease, which may inhibit the oxidation or reduction reaction of the catalyst. In one embodiment, the ratio of the thickness of the substrate to the thickness of the catalyst layer can specifically be 50:1 to 800:1, and more specifically 100:1 to 500:1.

[0041] According to another aspect of the present disclosure, an electrochemical cell including the aforementioned electrodes as a cathode and an anode is provided. Since both the cathode and the anode of the electrochemical cell contain a single-atom catalyst, it can have advantages in an electrochemical advanced oxidation process. Specifically, the single-atom catalyst of the anode exhibits high activity for generating hydroxyl radicals by the oxidation reaction of water, and the single-atom catalyst of the cathode exhibits high activity for the selective oxygen reduction reaction for generating hydrogen peroxide, which is a precursor of hydroxyl radicals. That is, since both the cathode and the anode of the electrochemical cell contain a single-atom catalyst, it can have high hydroxyl radical generation efficiency. Hydroxyl radicals act as an oxidizing agent for organic pollutants. As a result, the electrochemical cell is excellent in the decomposition efficiency of organic pollutants.

[0042] According to another aspect of the present disclosure, an electrochemical water treatment method using the aforementioned electrochemical cell is provided.

[0043] As described above, since both the cathode and the anode of the electrochemical cell contain a single-atom catalyst, it is excellent in the decomposition efficiency of organic pollutants.

[0044] Not only that, when using a normal electrochemical cell including electrodes such as boron-doped diamond (BDD) and lead oxide (PbO 2 ) as an anode, the formation of hydroxyl radicals at the anode is possible only when the pH of the wastewater to be treated by the oxidation reaction of water generated from the anode is about 3 to 4, and there is a drawback that it is difficult to treat wastewater having other pH values. The electrochemical cell according to the present disclosure has an advantage that it can treat wastewater having a wider range of pH values because the anode contains a single-atom catalyst. Specifically, the electrochemical water treatment using the electrochemical cell can be performed at a pH of about 3 to 9.

[0045] In addition, the cathode of the electrochemical cell of the present disclosure contains a single-atom catalyst, which has the advantage of being able to further enhance the efficiency of hydroxyl radical generation because the reactivity of the selective oxygen reduction reaction for hydrogen peroxide generation is high compared to a normal electrochemical cell.

[0046] Hereinafter, embodiments of the present invention will be further described with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are merely illustrative of the present invention and do not limit the appended claims. It is obvious to those skilled in the art that various changes and modifications to the examples are possible within the scope of the present invention and the scope of the technical idea, and it is natural that those modifications and variations also belong to the appended claims.

[0047] Production Example 1: Production of Single-Atom Catalyst (Co 1 -NCB) Cobalt(II) chloride hexahydrate (CoCl 2 ·6H 2 O) and 1,10-phenanthroline were dissolved in pure ethanol at a molar ratio of 1:3. The suspension obtained by mixing this with carbon black powder was shaken overnight and then dried at a temperature of 80°C. The powder obtained after drying was calcined at 600°C for 2 hours using a tubular furnace (SH-FU-80STG, SH Scientific, Korea) under an Ar atmosphere, and the temperature of the furnace was raised at a heating rate of 10°C / min until it reached 600°C. Finally, Co 1 -NCB powder was obtained (hereinafter referred to as 1wt% Co 1 -NCB). The Co 1 -NCB powder production process is schematically shown in Figure 1a. The produced Co 1 -NCB powder was cooled and stored in an Ar atmosphere until further use. Co 1 -NCB with a higher Cowt% was synthesized by a similar process except that a large amount of cobalt(II) chloride hexahydrate and 1,10-phenanthroline were used compared to 1wt% Co 1 -NCB at the same molar ratio (hereinafter referred to as 3wt% Co 1 -NCB and 5wt% Co 1-NCB.

[0048] Production Example 2: Co 1 Manufacture of -NCB electrode Co 1 The manufacturing process of the -NCB electrode is shown in Fig. 1b. 20 mg of Co 1 -NCB powder, 2 mL of IPA (isopropanol), and 80 μL of Nafion (Nafion® 117) were mixed to produce catalyst ink. Nafion® 117 refers to tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid copolymer. Nafion 5% wt refers to a solution of 5 wt% Nafion polymer in a solvent of isopropanol and water. The Nafion polymer is a trademark product sold by Chemours Chemical Company and is a polymer with a hydrophobic backbone of PTFE and a hydrophilic side chain containing sulfonic acid groups (-SO 3 H). After ultrasonic treatment of the catalyst ink for 2 hours, it was spray-coated on an electrode substrate (SUS304) with a geometric area of 1.0 × 1.0 cm 2 . The catalyst ink spray-coated on each electrode substrate was 0.5 ml. Both the anode and the cathode were coated in the same way.

[0049] Experimental Example 1: Co 1 Measurement of surface characteristics of Co-NCB catalyst Co of Production Example 1 1The morphology of -NCB was analyzed using a 200 kV aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM, JEM134 ARM200CF). X-ray diffraction (XRD, Rigaku) analysis was performed using Cu Kα radiation (λ = 1.54 Å) in the 2θ range of 10 - 80°. Co K-edge XANES (X-ray absorption near-edge structure) and EXAFS (Extended x-ray absorption fine structure) spectra were measured at the Materials Measurement 6-BM (BMM) beamline for the NSLS-II at Brookhaven National Laboratory (BNL). The measurements were performed in fluorescence mode using a four-element silicon drift detector (SDD). A Co foil was measured simultaneously and used for spectral alignment. Cobalt oxide (CoO) and cobalt phthalocyanine (Co-Pc) were used as reference materials. The pre-edge and post-edge backgrounds were removed, and the spectra were normalized to 1 based on the edge height. Data reduction was performed using Athena, and EXAFS fitting was performed using Artemis of the Demeter package. Wavelet transform analysis was performed using the Larch package with a Continuous Cauchy mother wavelet.

[0050] Figure 2a shows the Co 1-HAADF-STEM images of NCB, where Figs. 2b and 2c are EDS element maps of carbon and cobalt, respectively. Figs. 2a - 2c show that carbon and cobalt elements are relatively uniformly distributed on a spherical carbon black support with an average diameter of about 30 nm. Figs. 2d - 2f are HAADF-STEM images of Co single atoms on carbon black taken at various magnifications. Figs. 2d - 2f show that Co does not aggregate into metal clusters or particles. In contrast, the part indicated by the circle measuring only a few Å (angstroms) in Fig. 2f shows that Co is atomically fixed on the carbon black support.

[0051] Figs. 3a and 3b are XRD diffraction patterns, which indicate that there are not enough crystalline Co species in Co 1 -NCB. The two broad diffraction peaks at about 24° and 43° correspond to the (002) and (101) planes of carbon black (JCPDS card No. 34 - 0567).

[0052] Referring to Fig. 3b, when the Co loading exceeds 1 wt%, diffraction peaks characteristic of both metallic and metal oxide forms of Co appear. Also, the decrease in XRD peak intensity and the broadening of the CB peak at Co loadings of 3 wt% and 5 wt% indicate the formation of metallic Co and metal oxide nanoparticles, which is consistent with the transmission electron microscope (TEM) images.

[0053] Fig. 4a is the normalized Co K-edge XANES spectra for Co 1 -NCB, Co-foil, CoO, and Co-Pc, and Fig. 4b is the Fourier-transformed k 1 for Co 2It is a weighted EXAFS spectrum. Stronger evidence supporting the atomic dispersion of Co in NCB was obtained by XANES and EXAFS analyses. The spectrum in Figure 4a consists of the following three regions: i) weak pre-edge features related to forbidden electronic transitions from the 1s to the 3d levels, ii) the main absorption edge related to electronic transitions from the occupied 1s level to the empty valence 4p band, which provides a good estimate of the formal oxidation state of Co, and iii) extended EXAFS that can be used to determine the local structure around the absorbing atom. Co 1 - The XANES of NCB indicates that the edge is between that of Co foil and CoO reference, which implies a formal oxidation state between 0 and +2. 5 wt% Co 1 - The NCB spectrum closely follows that of Co foil and has a shoulder at about 7712 eV. 3 wt% Co 1 - NCB has a smaller shoulder at 7712 eV. In contrast, 1 wt% Co 1 - The XANES spectrum for NCB shows little shoulder similar to the CoO reference.

[0054] Referring to Figure 4b, for 3 wt% and 5 wt% Co 1 - The EXAFS spectrum of NCB closely matches that of Co foil, while for 1 wt% Co 1 - It can be confirmed that the EXAFS of NCB is in good agreement with the spectra of CoO and Co-Pc.

[0055] Figures 4c and 4d are the EXAFS fitting of the first shell of 1 wt% Co 1 Referring to Figures 4c and 4d, for 1 wt% Co 1 - NCB shows a Co-N / O path length of 1.98 Å in the first shell, which is longer than the Co-N path of Co-Pc. The presence of the Co-O path and a coordination number of 4.96 for 1 wt% Co is similar to CoO 1 - This supports the XANES results of NCB.

[0056] For 3 wt% and 5 wt% of Co 1-All NCBs have Co-Co in the first shell, which indicates a metal cluster. This result is consistent with previous research findings on Co nanoparticles.

[0057] Figures 4e to 4h are the results of wavelet (waelet) transform (WT) analysis of χ(k) for 1 wt% Co 1 -NCB, Co-Pc, CoO, and Co foil. As shown in Figures 4e to 4h, for 1 wt% Co 1 -The maximum intensity of the WT profile of NCB appears at approximately 4.9 Å, which is in close agreement with Co-Pc and CoO -1 This indicates that there is a first shell consisting of Co-N / O. For 3 wt% and 5 wt% Co 1 -NCB, the contour maximum intensity is shown at approximately 7.5 Å -1 and is similar to that of Co foil, implying that Co forms coordination bonds with other Co atom-forming clusters. Table 1 below shows the EXAFS fitting parameters at the Co K-edge.

[0058]

Table 1

[0059] Here, R is the interatomic distance (bond length between the central atom and the surrounding coordination atoms), and σ 2 is the Debye-Waller factor, and the R-factor indicates the goodness of fit. S 0 2 (amplitude reduction factor) was estimated by fitting Co foil. A value of 0.768 was set for the EXAFS fit of the sample. * indicates that the coordination number is fixed to the known crystallographic value for the reference compound.

[0060] Co with different Co loadings 1The oxidation state change of the metal site of -NCB was further investigated via soft XAS. Compared with the above-mentioned XANES and EXAFS (hard XAS), soft XAS provides more surface-concentrated information via the Co L-edge spectrum (the conversion of the 2p electron to the unfilled d orbital). Referring to the soft XAS plot in Figure 5, 1 wt% Co 1 -NCB indicates that the oxidation state of Co is between 0 and +2, which supports the existence of Co in a single-atom structure. According to the above results, in Co 1 -NCB with a high Co loading, Co atoms tend to aggregate into clusters or nanoparticles during the annealing process due to excess Co atoms and insufficient spacing between them. Considering this point, in the subsequent experimental examples, Co with a 1.0 wt% Co loading 1 -NCB electrodes were only considered.

[0061] Experimental Example 2: Production of hydroxyl radicals in 1 wt% Co 1 -NCB anodes The production of reactive oxygen species (ROS) was monitored in an H cell separated by a Nafion 117 membrane (a perfluorosulfonic acid membrane made of a sulfonated tetrafluoroethylene copolymer). Electron paramagnetic resonance (EPR) spectroscopy (JES-X310, JEOL) was performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap agent to identify ROS. Samples were collected at predetermined time intervals and analyzed under the following conditions: microwave frequency = 9,417 MHz; microwave power = 5 mW; modulation frequency = 100 MHz; modulation amplitude = 2.0 G.

[0062] Electrically generated hydroxyl radicals are predicted by the water oxidation reaction (H 2 O → ·OH + H + + e - ). 1 wt% Co 1Other reactive oxygen species involving the generation of hydroxyl radicals at the -NCB anode were analyzed using electron paramagnetic resonance (EPR) spectroscopy. A 1:2:2:1 quartet, characteristic of the DMPO-·OH adduct, was observed at the Co 1 -NCB anode after 10 minutes of electrolysis. As shown in Fig. 6, the intensity of the adduct signal gradually increased over 30 minutes.

[0063] Fig. 7a shows the EPR spectrum of the DMPO adduct as a function of current density, and Fig. 7b shows the EPR spectrum of the DMPO adduct as a function of initial pH. Referring to Fig. 7a, as the current density increased, the EPR signal intensity of the radicals increased. Also, referring to Fig. 7b, unlike previous studies suggesting that more hydroxyl radicals are formed under acidic conditions, the Co 1 -NCB anode showed consistent generation of hydroxyl radicals over a wide range of solution pH values from 3 to 9. On the other hand, the 1:1:1:1 quadruplet peak of the DMPO-·O 2 - adduct was not observed. This indicates that there is no competitive electrogeneration of other minor oxidative species such as O 1 at the Co 2 - -NCB anode.

[0064] Experimental Example 3: Production of hydrogen peroxide at the 1 wt% Co 1 -NCB cathode Transition metal single-atom catalysts have been reported to effectively promote the production of H 2 +2e - +2H + →H 2 O 2 via the two-electron selective oxygen reduction reaction (O 2 O 2 ). Therefore, the electrocatalytic performance of the Co 1 -NCB cathode in the two-electron selective oxygen reduction reaction and the activation of H 2 O 2 for hydroxyl radical production were further investigated.

[0065] H in solution2 O 2 The concentration was quantified by iodine titration using a UV-vis spectrophotometer (Biotek Synergy Mx). Briefly, 1 mL of the sampled amount was mixed with 1 L of deionized water, 0.75 mL of potassium hydrogen phthalate (0.1 M), 0.75 mL of potassium iodide (0.4 M), NaOH (0.06 M), and 10 -4 M), stirred vigorously for 2 minutes, and then the absorbance was measured at 348 nm.

[0066] H 2 O 2 The test strip was used to monitor the H 2 O 2 concentration during electrolysis. Figures 8a and 8b show that the test strip changed to blue after 10 minutes of the reaction, indicating that H 1 was generated by the selective oxygen reduction reaction at the Co 2 O 2 -NCB cathode. Figures 9a and 9b are plots of the change in absorbance with respect to a specific wavelength over time and the change in hydrogen peroxide concentration over time, respectively. Referring to Figure 9b, the H 2 O 2 concentration first increased, reached a maximum of approximately 1 mg / L after 10 minutes, and then decreased. This was consistent with the colorimetric results in Figure 9a.

[0067] Co 1 Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements of the -NCB cathode were performed using a potentiostat (Ivium Technologies of the Netherlands). All measurements were carried out in a three-electrode electrochemical cell consisting of a platinum (Pt) mesh as the counter electrode, an Ag / AgCl as the reference electrode, and Co 1 -NCB as the working electrode.

[0068] To evaluate the oxygen reduction reaction activity of the cathode, O 2 - and N 2- Saturated 100 mM NaClO 4 Electrochemical studies were conducted with electrolytes. CV was performed at a scan rate of 10 mV / s in an applied voltage window of +1.0 V to -1.5 V (versus Ag / AgCl).

[0069] Interestingly, H 2 O 2 is generated only under saturated conditions at the Co 2 -NCB cathode of the H cell. This suggests that it occurs by consuming oxygen to generate H 1 in the cathode chamber for the oxygen reduction reaction. In Figure 10, this was verified by detecting H 2 O 2 in a single cell where the oxygen generated at the Co 1 -NCB anode migrated to the Co 1 -NCB cathode. Figure 8c shows the comparative LSV profile of the Co 2 O 2 -NCB cathode in 100 mM NaClO 2 / N 2 saturated electrolyte, and the small graph corresponds to the CV curve. Referring to Figure 8c, during CV analysis, the occurrence of a reduction current peak at approximately -0.8 V in the O 4 saturated environment enables further confirmation of the synthesis of H 1 from the oxygen reduction reaction. 2 O 2 O 2

[0070] Experimental Example 4: Electrochemical Oxidation of Phenol Co 1 -NCB anodes and cathodes electrochemically and catalytically generated hydroxyl radicals can effectively decompose organic pollutants present in water, and this was used to evaluate the performance of the Co 1 -NCB electrode in an electrochemical advanced oxidation process.

[0071] Electrochemical phenol oxidation was carried out with 100 mM of NaClO 4 ​Performed using a 60 mL single-compartment cell containing an electrolyte (pH = 6.5, conductivity = 9.91 mS / cm). The electrochemical cell was equipped with a Co 2 (partitioned using Teflon®) electrode with a defined geometric area of 1.0 cm 1 -NCB. The distance between the electrodes was 1.0 cm. The experiment was carried out in a potentiostatic mode using a potentiostat (Ivium Technologies, Netherlands) while applying a constant current density of 5 - 20 mA / cm 2 at room temperature under magnetic stirring (500 rpm).

[0072] Figures 11a and 11b are plots showing the electrochemical oxidation efficiency of phenol using the Co 1 -NCB electrode at different current densities and different initial pH values, and Figure 11c is a comparative graph of the phenol removal efficiency when both the cathode and anode are Co 1 -NCB electrodes and when only the anode is a Co 1 -NCB electrode.

[0073] Referring to Figure 11a, as the current density increased from 5 mA / cm 2 to 20 mA / cm 2 , the phenol removal efficiency increased. Also, phenol removal at various current densities was suitable for pseudo-first order kinetics with rate constants of 0.398, 0.664, 0.794, and 0.832 h -1 (R 2 > 0.99 for each case). The most important side reactions that can reduce the generation of hydroxyl radicals are (i) the generation of H 2 at the cathode (2H + + 2e - → H 2 ) and (ii) the generation of O 2 at the anode (2H 2 O → O 2 + 4H + + 4e - ), which can compete with the production of hydroxyl radicals. However, at a current density of 20 mA / cm 2Up to this point, a decrease in the phenol removal efficiency due to the hydrogen generation side reaction at the cathode was not observed, which is presumably due to the generation of H 2 O 2 by the oxygen reduction reaction at the cathode and the radical generation by its activation. Another side reaction, the oxygen evolution reaction at the anode, was confirmed not to suppress radical generation with a Co 2 -NCB anode up to a current density of 20 mA / cm 1 .

[0074] Figure 11b shows the electrochemical phenol removal of two Co 1 -NCB electrodes at initial pH values of 3.0, 6.5, and 9.0. By linear regression analysis of the phenol removal over time during the electrochemical oxidation process, a pseudo-first-order model (R 2 > 0.99) was confirmed. The phenol removal efficiency was highest at pH 6.5. However, in all cases, the phenol removal efficiency exceeded 90% within 240 minutes. This means that the Co 1 -NCB electrode-based electrochemical advanced oxidation process has high electrochemical activity over a wide pH range, which is also consistent with the EPR results.

[0075] Such results mean that radical generation and phenol removal in the Co 1 -NCB electrode system show similar trends. Therefore, the influence of the Co 1 -NCB electrode configuration on radical generation was evaluated based on the phenol removal efficiency. Referring to Figure 11c, it was observed that when the Co 1 -NCB anode is paired with the Co 1 -NCB cathode, there is a significant improvement in both the phenol removal efficiency and the kinetic constant compared to a cell where the Co 1 -NCB anode is paired with a stainless steel cathode. This comparison shows that the Co 1 -NCB cathode contributes significantly in the electrochemical advanced oxidation process. Essentially, Co 1- The NCB cathode enhances the oxidation ability of the system by improving the production of hydroxyl radicals through the cathode oxygen reduction reaction.

[0076] The above description is merely an exemplification applying the principles of the present disclosure and may further include other configurations without departing from the scope of the present invention.

Claims

1. 1. A single atom catalyst comprising: a nitrogen-doped carbon structure; and a monatomic metal, The monoatomic metal forms a coordinate bond with a nitrogen atom of the nitrogen-doped carbon structure.

2. 2. The single atom catalyst of claim 1, wherein the carbon structure is carbon black.

3. 2. The single atom catalyst of claim 1, wherein the single atom metal is cobalt (Co).

4. 2. The single atom catalyst of claim 1, wherein the single atom metal content in the single atom catalyst is greater than 0 wt % and less than 3 wt %.

5. 1. A method for producing a single atom catalyst, comprising the steps of: mixing a monoatomic metal precursor and a nitrogen dopant precursor in a solvent to produce a mixed solution; mixing the carbon structure powder into the mixed solution to prepare a suspension; shaking and drying the suspension to produce a single atom catalyst precursor; and calcining the single atom catalyst precursor to produce a single atom catalyst.

6. An electrode comprising: A substrate; a catalyst layer coated on the substrate; An electrode, wherein the catalyst layer comprises the single-atom catalyst according to any one of claims 1 to 4.

7. The electrode of claim 6 , wherein the substrate comprises stainless steel.

8. 7. The electrode of claim 6, wherein the ratio of substrate thickness to catalyst layer thickness is from 10:1 to 1000:

1.

9. 7. An electrochemical cell comprising the electrodes of claim 6 as a cathode and an anode.

10. An electrochemical water treatment method using the electrochemical cell according to claim 9.

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