Method for producing oxygen reduction reaction catalyst and oxygen reduction reaction catalyst

Abstract

[Problem] To provide a method for producing an oxygen reduction reaction catalyst having relatively superior characteristics not only under alkaline conditions but also under neutral and acidic conditions, and to provide an oxygen reduction reaction catalyst. [Solution] The method comprises: a preliminary carbonization treatment step for obtaining rice husk biochar by putting a raw material containing rice husks into a solution containing iron and sulfuric acid, and performing hydrothermal carbonization treatment; a heat treatment step for mixing the rice husk biochar obtained in the preliminary carbonization treatment step with sodium chloride and urea, and then heating the mixture for heat treatment in a nitrogen atmosphere; and a neutralization step for obtaining an oxygen reduction reaction catalyst by subjecting the product obtained in the heat treatment step to acid treatment for neutralization.

Description

Method for producing oxygen reduction reaction catalyst and oxygen reduction reaction catalyst

[0001] The present invention relates to a method for producing an oxygen reduction reaction catalyst and an oxygen reduction reaction catalyst.

[0002] Fuel cells have been attracting attention as a next-generation energy source that will replace fossil fuel cells, and high-performance platinum catalysts with four electron transfer pathways are widely used as cathode catalysts in fuel cells. However, because platinum and platinum group metals (PGMs) are expensive and rare, the development of alternative catalysts is underway.

[0003] In the past, due to environmental concerns, biomass has been considered as a raw material for catalysts instead of platinum catalysts. An oxygen reduction reaction (ORR) catalyst using rice husk (RH), an agricultural waste, as a cathode catalyst has been developed (see, for example, Non-Patent Document 1). This catalyst is manufactured as follows: First, dried rice husks are mixed with a ZnCl2 activator and heated at 900°C for 2 hours to obtain Si-self-doped activated rice husk charcoal (Si-RH-x). This rice husk charcoal is then mixed with (NH)2Fe(SO4)2 and heated at 800°C for 2 hours for pyrolysis to obtain an oxygen reduction reaction catalyst containing Fe, S, and N (Si-Fe / S / N / -RH-x).

[0004] The present inventors have developed a method for producing highly crystalline carbon using biomass as a raw material (see, for example, Patent Document 1). In this method, a raw material containing biomass is first impregnated with at least one of Fe, Ni, and Co, and then heat-treated in a closed system at 100 to 500°C to obtain a precursor. The precursor is then heated to 350°C or higher to carbonize it, and then acid-washed to produce highly crystalline carbon.

[0005] Fuling Wang et al., “Conversion of rice husk biomass into electrocatalyst for oxygen reduction reaction in Zn-air battery: Effect of self-doped Si on performance”, Journal of Colloid and Interface Science, 2022, Vol. 606, Part 2, p.1014-1023

[0006] Japanese Patent Application Laid-Open No. 2022-126560

[0007] The oxygen reduction reaction catalyst described in Non-Patent Document 1 exhibits superior properties to conventional PtC catalysts and iron-based catalysts under alkaline conditions, but has the problem of inferior properties under neutral and acidic conditions.

[0008] The present invention has been made in light of the above-mentioned problems, and an object of the present invention is to provide a method for producing an oxygen reduction reaction catalyst, and an oxygen reduction reaction catalyst, which have relatively excellent properties not only under alkaline conditions but also under neutral and acidic conditions.

[0009] In order to achieve the above-mentioned object, the method for producing an oxygen reduction reaction catalyst according to the present invention is characterized by comprising a preliminary carbonization process in which a raw material containing rice husks is placed in a solution containing iron and sulfuric acid and subjected to hydrothermal carbonization to obtain rice husk charcoal; a heat treatment process in which sodium chloride and urea are added to the rice husk charcoal obtained in the preliminary carbonization process, mixed, and then heated in a nitrogen atmosphere to perform heat treatment; a neutralization process in which the product obtained in the heat treatment process is neutralized by performing an acid treatment; and a secondary carbonization process in which the product obtained in the neutralization process is heated in a nitrogen atmosphere to perform heat treatment to obtain an oxygen reduction reaction catalyst.

[0010] The method for producing an oxygen reduction reaction catalyst according to the present invention is environmentally friendly, allowing the use of rice husks, which have traditionally been discarded, as a catalyst raw material instead of expensive and rare platinum and platinum group metals (PGMs). Furthermore, the method for producing an oxygen reduction reaction catalyst according to the present invention allows the addition of Fe in the preliminary carbonization process and N in urea in the heat treatment process, in addition to the C and Si contained in rice husks. Furthermore, by adding sodium chloride in the heat treatment process, a more rugged, porous, and irregular structure can be obtained. This allows the oxygen reduction reaction to be promoted via a four-electron transfer pathway, and oxygen reduction reaction catalysts with relatively excellent properties can be produced not only under alkaline conditions but also under neutral and acidic conditions. Furthermore, by performing another heat treatment after the neutralization treatment, oxygen reduction reaction catalysts with even better properties can be obtained.

[0011] The method for producing an oxygen reduction reaction catalyst according to the present invention preferably includes a pretreatment step of powdering pyrite and performing hydrothermal treatment to obtain the solution containing iron and sulfuric acid, and the preliminary carbonization step preferably includes performing hydrothermal carbonization using the solution obtained in the pretreatment step. The growth of the electric vehicle industry in recent years has led to increased demand for copper, which has resulted in an increase in the amount of pyrite generated as waste. Therefore, by using pyrite in the pretreatment step, discarded pyrite can be effectively utilized, which is environmentally friendly.

[0012] In the method for producing an oxygen reduction reaction catalyst according to the present invention, the preliminary carbonization step preferably involves hydrothermal carbonization treatment under temperature and pressure conditions of 150°C to 300°C and 1 MPa to 10 MPa for 1 to 8 hours, and more preferably hydrothermal carbonization treatment under 1 MPa to 3 MPa for 4 to 8 hours. The heat treatment step preferably involves heat treatment under a nitrogen atmosphere at 700°C to 1000°C for 1 to 3 hours. The secondary carbonization step preferably involves heat treatment under a nitrogen atmosphere at 750°C to 1050°C for 0.5 to 2 hours. In these cases, an oxygen reduction reaction catalyst with better properties can be produced. Furthermore, by performing heat treatment at 700°C to 900°C in the heat treatment step, an oxygen reduction reaction catalyst with even better properties can be produced.

[0013] In the method for producing an oxygen reduction reaction catalyst according to the present invention, the pretreatment step preferably involves hydrothermal treatment for 0.5 to 2 hours under temperature and pressure conditions of 150°C to 200°C and 0.8 MPa to 5 MPa, and more preferably hydrothermal treatment at 0.8 MPa to 1.2 MPa. In these cases, a solution containing iron and sulfuric acid can be efficiently produced from pyrite, which is the raw material for the pretreatment step.

[0014] The oxygen reduction reaction catalyst according to the present invention is characterized by being porous, containing C, N, Fe, and Si, and being in the form of uniform, wrinkled carbon nanoflakes.

[0015] The oxygen reduction reaction catalyst according to the present invention is suitably produced by the method for producing an oxygen reduction reaction catalyst according to the present invention. The oxygen reduction reaction catalyst according to the present invention can promote an oxygen reduction reaction via a four-electron transfer pathway and has relatively excellent properties not only under alkaline conditions but also under neutral and acidic conditions.

[0016] The oxygen reduction reaction catalyst according to the present invention preferably has the N and Fe added to rice husk biochar, and preferably has a site where the Fe is bonded to the N and amorphous silica. In these cases, more excellent properties can be obtained. Rice husk biochar is a carbide made from rice husks.

[0017] The fuel cell cathode according to the present invention is characterized by containing the oxygen reduction reaction catalyst according to the present invention. When used as a fuel cell cathode, the fuel cell cathode according to the present invention has, for example, an excellent discharge rate as well as long-term durability and reversibility, thereby achieving excellent power generation performance. The fuel cell cathode according to the present invention may contain a mixture of the oxygen reduction reaction catalyst and a conductive additive to further improve power generation performance and durability. The mixing ratio (mass ratio) of the oxygen reduction reaction catalyst to the conductive additive is preferably 3:1 to 5:1. The conductive additive is preferably graphite or white charcoal derived from biomass, and more preferably carbon black.

[0018] According to the present invention, it is possible to provide a method for producing an oxygen reduction reaction catalyst and an oxygen reduction reaction catalyst that have relatively excellent properties not only under alkaline conditions but also under neutral and acidic conditions.

[0019] 1 is a perspective view showing (a) a pretreatment step, (b) a preliminary carbonization step, a heat treatment step, and a neutralization step in a method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention. (a) Scanning electron microscope (SEM) photographs of rice husk charcoal (Pristine RH), (b) comparative rice husk charcoal, and (c) the resulting oxygen reduction reaction catalyst (RH-Si-Fe-NC) in the method for producing the oxygen reduction reaction catalyst shown in Fig. 1. (a) A transmission electron microscope (TEM) image of the resulting oxygen reduction reaction catalyst, and (b) elemental mapping of C, N, Fe, and Si by energy dispersive X-ray spectroscopy (EDS), (c) a TEM image of rice husk charcoal obtained after the preliminary carbonization step, and (d) elemental mapping of C, N, Fe, and Si by EDS in the method for producing the oxygen reduction reaction catalyst shown in Fig. 1. 1 shows XRD spectra of (a) the oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the method for producing the oxygen reduction reaction catalyst shown in Fig. 1, (b) rice husk charcoal (pristine RH), and (c) SiC, (d) Fe3C, (e) SiO2, (f) graphite carbon, and (g) Fe2O3 for reference. 1 shows Raman spectra of (a) the oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the method for producing the oxygen reduction reaction catalyst shown in Fig. 1, and (b) rice husk charcoal (pristine RH). 1 shows (a) BET adsorption isotherms, (b) pore size distributions, and (c) DFT pore size distributions of an oxygen reduction reaction catalyst (RH-Si-Fe-NC-850°C) obtained at a heating temperature of 850°C in the heat treatment step, an oxygen reduction reaction catalyst (RH-Si-Fe-NC-950°C) obtained at a heating temperature of 950°C in the heat treatment step, and rice husk charcoal (pristine RH) in the method for producing the oxygen reduction reaction catalyst shown in Fig. 1. XPS (X-ray photoelectron spectroscopy) shows (a) Su1s spectrum, (b) N1s spectrum, and (c) Fe2p spectrum of the oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (pristine RH) obtained by the method for producing the oxygen reduction reaction catalyst shown in Fig. 1.X-ray absorption fine structure (XAFS) spectra of the oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the method for producing the oxygen reduction reaction catalyst shown in Figure 1, and FePc, Fe2O3, FeO, and Fe_Foil as references, show (a) XAFS spectra by Fe K-edge XANES and extended X-ray absorption fine structure (EXAFS), and (b) radial distance distribution functions by Fourier transform of the EXAFS spectra. XAFS spectra of the oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the method for producing the oxygen reduction reaction catalyst shown in Figure 1, and amorphous SiO2, SiO2 (Silica), FeSiO4, SiO, and pure Si as references, show (a) XAFS spectra by Si K-edge XANES and EXAFS, and (b) radial distribution functions by Fourier transform of the EXAFS spectra. The oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the heat treatment process at 850°C (RHN-(@)750°C), the oxygen reduction reaction catalyst (RHN-(@)950°C) obtained by the heat treatment process at 750°C (RHN-(@)750°C), and the oxygen reduction reaction catalyst (RHN-(@)950°C) obtained by the heat treatment process at 950°C (RHN-(@)950°C) in the method for producing the oxygen reduction reaction catalyst shown in Figure 1 , as well as rice husk charcoal (Pristine RH), and a commercially available PtC catalyst, are shown in (a) and (b) below, respectively, as determined by linear sweep voltammetry (LSV) under alkaline conditions. 1 / 2 and limiting current density J ｋ1 , (a) the number of electron transfer and hydrogen peroxide yield measured by LSV under alkaline conditions for the oxygen reduction reaction catalyst (RH-Si-Fe-NC) obtained by the heat treatment process at 850°C (RHN-750°C), the oxygen reduction reaction catalyst obtained by the heat treatment process at 750°C (RHN-750°C), the oxygen reduction reaction catalyst obtained by the heat treatment process at 950°C (RHN-950°C), and a commercially available PtC catalyst. (b) the durability test results. (c) a voltammogram when KCN was added to the electrolyte. (a) LSV voltammogram and (b) half-wave potential E of the oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (pristine RH) obtained by the oxygen reduction reaction catalyst production process shown in FIG. 1 , and a commercially available PtC catalyst under neutral conditions. 1 / 2 and limiting current density J ｋ 1 , (c) Tafel slope. (a) LSV electron transfer number and hydrogen peroxide yield, (b) durability test results, and (c) durability test results when methanol was added to the electrolyte solution. (b) Open-circuit voltage, (c) constant-current discharge voltage, and (c) polarization curves during discharge are shown for electrodes containing the oxygen reduction catalyst (RH-Si-Fe-NC) obtained by the oxygen reduction catalyst manufacturing method shown in FIG. 1 and rice husk charcoal (pristine RH), and a commercially available PtC catalyst, measured under neutral conditions. The oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH) obtained by the method for producing the oxygen reduction reaction catalyst shown in FIG. 1 , as well as a commercially available PtC catalyst, were analyzed under acidic conditions. (a) LSV voltammograms and (b) half-wave potentials E 1 / 2 and limiting current density J ｋ2A and 2B are graphs showing LSV voltammograms of electrodes containing only the oxygen reduction reaction catalyst (RH-Fe-N) obtained by the method for producing the oxygen reduction reaction catalyst shown in FIG. 1 , a mixture of the oxygen reduction reaction catalyst (RH-Fe-N) and commercially available carbon black at a 4:1 ratio (RH-Fe-N-CB), rice husk charcoal (Pristine RH) only, a mixture of rice husk charcoal (Pristine RH) and commercially available carbon black at a 4:1 ratio (Pristine RH CB), and a carbon catalyst (PtC 40 wt.%) only, under (a) alkaline conditions, (b) neutral conditions, and (c) acidic conditions. 2 is a graph showing the Tafel slopes under acidic, neutral, and alkaline conditions for electrodes containing only the oxygen reduction reaction catalyst (RH-Fe-N) obtained by the method for producing the oxygen reduction reaction catalyst shown in Fig. 1, a mixture of the oxygen reduction reaction catalyst (RH-Fe-N) and commercially available carbon black at a 4:1 ratio (RH-Fe-N-CB), and a carbon catalyst (PtC 40 wt.%) only. FIG. 3 is a graph showing the results of LSV durability tests under (a) neutral and (b) acidic conditions for electrodes containing only the oxygen reduction reaction catalyst (RH-Fe-N) obtained by the method for producing the oxygen reduction reaction catalyst shown in Fig. 1, a mixture of the oxygen reduction reaction catalyst (RH-Fe-N) and commercially available carbon black at a 4:1 ratio (RH-Fe-N-CB), and a carbon catalyst (PtC 40 wt.%) only.

[0020] Hereinafter, embodiments of the present invention will be described with reference to the drawings and examples. Figures 1 to 18 show a method for producing an oxygen reduction reaction catalyst and the oxygen reduction reaction catalyst according to an embodiment of the present invention. As shown in Figure 1, the method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention includes a pretreatment step, a preliminary carbonization step, a heat treatment step, a neutralization step, and a secondary carbonization step.

[0021] As shown in Figure 1(a), the pretreatment process begins by placing powdered pyrite in water and subjecting it to hydrothermal dissolution at a predetermined temperature and pressure while stirring in a hydrothermal treatment device whose interior is purged with oxygen (O2). In one specific example shown in Figure 1(a), the hydrothermal dissolution is carried out for 0.5 to 2 hours at a temperature and pressure of 150°C to 200°C, 0.8 MPa to 5 MPa, and a stirring speed of 200 to 500 rpm. After the hydrothermal dissolution, filtration is performed to extract only the liquid.

[0022] The reaction in this hydrothermal decomposition process is shown below: FeS2 + 8H2O → Fe 2+ + 2SO4 2- + 16H + + 14e - As shown in this reaction formula, the extracted liquid is a solution containing iron and sulfuric acid (pyrite solution).

[0023] As shown in Figure 1(b), the preliminary carbonization process involves placing the rice husk-containing raw material in a solution containing iron and sulfuric acid, which was obtained in the pretreatment process, and then performing hydrothermal carbonization in a hydrothermal treatment device with the interior purged with nitrogen (N2). In a specific example shown in Figure 1(b), the hydrothermal carbonization process is carried out under temperature and pressure conditions of 150°C to 300°C, 1 MPa to 10 MPa, and a stirring speed of 200 to 500 rpm for 1 to 8 hours. Powdered rice husk charcoal (rice husk biochar) is obtained from the resulting rice husk charcoal, which is a carbonized product of rice husks and contains Si, Fe, and C.

[0024] As shown in Figure 1(b), the heat treatment process involves adding and mixing the rice husk charcoal obtained in the preliminary carbonization process with the neutral salts sodium chloride (NaCl) and urea, then placing the mixture in a heat treatment vessel with a nitrogen atmosphere and heating it using the template method to carbonize it (neutral salt template carbonization). In one specific example shown in Figure 1(b), the heat treatment is performed at a heating rate of 3°C to 10°C / min at 700°C to 1000°C for 1 to 3 hours. The nitrogen atmosphere is maintained inside the heat treatment vessel by flowing nitrogen (N2) at a flow rate of 100 ml / min to 300 ml / min.

[0025] As shown in Fig. 1(b), the neutralization step involves neutralizing the product (carbonized material) obtained in the heat treatment step by acid treatment. In a specific example shown in Fig. 1(b), the product is neutralized by placing it in 3M hydrochloric acid and stirring it at room temperature.

[0026] As shown in FIG. 1(b), in the secondary carbonization step, the product (neutralized product) obtained in the neutralization step is placed in a heat treatment vessel with a nitrogen atmosphere inside, and heat treatment (nitrogen annealing) is performed using the template method, similar to the heat treatment step. In a specific example shown in FIG. 1(b), the heat treatment is performed at 750°C to 1050°C for 0.5 to 2 hours. The nitrogen atmosphere is maintained inside the heat treatment vessel by flowing nitrogen (N2) at a flow rate of 100 ml / min to 300 ml / min. The heat treatment temperature is preferably higher than that in the heat treatment step. After the secondary carbonization step, an oxygen reduction reaction (ORR) catalyst is obtained.

[0027] Thus, the method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention can suitably produce the oxygen reduction reaction catalyst according to an embodiment of the present invention. The method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention can use rice husks, which have been mainly discarded, as a raw material for the catalyst instead of expensive and rare platinum and platinum group metals (PGMs), and is therefore environmentally friendly. Furthermore, since pyrite is used in the pretreatment process, discarded pyrite can be effectively utilized, and this is environmentally friendly.

[0028] Furthermore, in the method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention, in addition to the C and Si contained in rice husks, Fe can be added in the preliminary carbonization process, and N can be added in the heat treatment process. Furthermore, by adding sodium chloride in the heat treatment process, a more undulating, porous, and irregular structure can be obtained. This allows the oxygen reduction reaction to be promoted via a four-electron transfer pathway, and an oxygen reduction reaction catalyst with relatively excellent properties can be produced not only under alkaline conditions, but also under neutral and acidic conditions.

[0029] The obtained oxygen reduction reaction catalyst is porous and contains Si, Fe, N, and C. The oxygen reduction reaction catalyst is made from rice husk charcoal to which N and Fe have been added, and has sites where Fe is bonded to N and amorphous silica. The catalyst also has a uniform, wrinkled carbon nanoflake structure.

[0030] An oxygen reduction reaction catalyst was produced using the method for producing an oxygen reduction reaction catalyst according to the embodiment of the present invention shown in FIG. 1 , and its characteristics were investigated. The oxygen reduction reaction catalyst was produced under the following conditions: In the pretreatment step, hydrothermal decomposition treatment was performed for 1 hour at a temperature and pressure of 170°C and 1 MPa with an agitation speed of 300 rpm. In the preliminary carbonization step, hydrothermal carbonization treatment was performed for 6 hours at a temperature and pressure of 250°C and 2 MPa with an agitation speed of 300 rpm. In the heat treatment step, the heating rate was 5°C / min, and heat treatment was performed at 750°C, 850°C, or 950°C for 2 hours. The flow rate of nitrogen flowing into the heat treatment vessel was 300 ml / min. In the secondary carbonization step, heat treatment was performed for 1 hour at 850°C. The flow rate of nitrogen flowing into the heat treatment vessel was 300 ml / min.

[0031] The obtained oxygen reduction reaction catalyst (hereinafter referred to as "RH-Si-Fe-NC" or "RH-Si-Fe-NC") was subjected to various measurements to investigate its properties. For comparison, measurements were also performed on rice husk charcoal (hereinafter referred to as "Pristine RH"), which was obtained by hydrothermal carbonization of rice husk-containing raw materials without placing them in the solution (pyrite solution) obtained in the pretreatment process.

[0032] [Observation by Scanning Electron Microscope (SEM)] Rice husk charcoal (pristine RH) and oxygen reduction reaction catalyst (RH-Si-Fe-NC) were observed by scanning electron microscope (SEM). For comparison, a raw material containing rice husk was placed in the pyrite solution obtained in the pretreatment process together with urea, and then subjected to hydrothermal carbonization (hereinafter referred to as "comparative rice husk charcoal"). The observation results are shown in Figures 2(a) to 2(c), respectively. The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C in the heat treatment process.

[0033] The comparative rice husk charcoal shown in Figure 2(b) has a more undulating and looser morphology than the rice husk charcoal shown in Figure 2(a). However, the oxygen reduction reaction catalyst shown in Figure 2(c) was confirmed to have an even more undulating, porous, and irregular morphology than the comparative rice husk charcoal. The structure of this oxygen reduction reaction catalyst is thought to be due to the sodium chloride nanoreactors added during the heat treatment process. Furthermore, because the oxygen reduction reaction catalyst is porous, it is expected to increase the number of exposed active sites and improve transport efficiency during the oxygen reduction reaction.

[0034] [Observation by Transmission Electron Microscope (TEM)] The rice husk charcoal after the preliminary carbonization process and the oxygen reduction reaction catalyst (RH-Si-Fe-NC) were observed using a transmission electron microscope (TEM), and elemental analysis was performed using energy dispersive X-ray spectroscopy (EDS). The observation results are shown in Figures 3(a) to 3(d). The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C in the heat treatment process.

[0035] As shown in Figure 3(a), the oxygen reduction reaction catalyst was confirmed to be homogeneous, wrinkled, and porous. The pores in the oxygen reduction reaction catalyst are thought to promote mass transfer to the active sites. In contrast, as shown in Figure 3(c), no pores were observed in the rice husk charcoal after the preliminary carbonization process, confirming its dense structure.

[0036] 3(b) and (d), both the oxygen reduction reaction catalyst and the rice husk charcoal after the preliminary carbonization treatment process contained C, N, Fe, and Si, confirming the addition of Fe and N. Furthermore, these elements were more uniformly distributed in the oxygen reduction reaction catalyst than in the rice husk charcoal after the preliminary carbonization treatment process, which is thought to make the catalyst more stable as a catalyst.

[0037] [Crystal structure analysis by X-ray diffraction (XRD)] The oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH) were measured by X-ray diffraction. The XRD spectra are shown in Figures 4(a) and 4(b). The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C in the heat treatment process.

[0038] As shown in Figure 4(b), rice husk charcoal exhibited two broad peaks: one at 2θ = 22.0° and the other at 44.0°, corresponding to graphitic carbon. In contrast, as shown in Figure 4(a), the oxygen reduction catalyst exhibited many sharp peaks. For example, a sharp peak corresponding to SiO2 was observed at 2θ = 22.0°, the same position as rice husk charcoal; a peak corresponding to graphitic carbon was observed at 2θ = 26.0°; a peak corresponding to SiC was observed at 2θ = 35.0°; a peak corresponding to Fe3C was observed near 2θ = 45.0°; and a peak corresponding to Fe2O3 was observed near 2θ = 55.0°. The peaks corresponding to SiC and Fe3C are thought to be due to the active sites of the oxygen reduction catalyst.

[0039] [Raman Spectroscopic Analysis] The oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH) were analyzed by Raman spectroscopy. The Raman spectra are shown in Figures 5(a) and 5(b). The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C during the heat treatment process.

[0040] As shown in Figures 5(a) and (b), both of them show the D band (1360-1380 cm -1 around 1590-1615 cm -1A clear peak was confirmed in the G band. Since a clear peak was observed in the G band, it can be said that both the oxygen reduction reaction catalyst and the rice husk charcoal have good electrical conductivity.

[0041] From the results of the Raman spectrum, the ratio of the intensities of each band (I D / I G ) was calculated and is shown in Figures 5(a) and (b). D / I G The value of the oxygen reduction reaction catalyst was 1.001, while that of the rice husk charcoal was 0.979, confirming that the oxygen reduction reaction catalyst was larger. This means that the oxygen reduction reaction catalyst has more defects and more catalytic active sites.

[0042] [Measurement of adsorption isotherms and pore size distributions by BET method, and pore size distributions by DFT method] For the oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (pristine RH), adsorption isotherms and pore size distributions were measured by BET method using N2 gas adsorption (77 K isotherm), and pore size distributions were measured by DFT method. The results by BET method are shown in Figures 6(a) and (b), and the results by DFT method are shown in Figure 6(c). The oxygen reduction reaction catalyst was manufactured at a heating temperature of 850°C or 950°C in the heat treatment process, and the results at each temperature are shown.

[0043] As shown in Figure 6(a), nitrogen uptake was clearly observed at relatively low pressures in all the samples, confirming the presence of micropores. Furthermore, as shown in Figure 6(b), for the oxygen reduction reaction catalysts, regardless of the heat treatment temperature, the pore volume rapidly decreased as the pore width increased, and the pore volume was particularly small for the catalyst heat-treated at 950°C. This is thought to be due to the fact that the micropores become more likely to collapse as the heat treatment temperature increases.

[0044] As shown in Figure 6(c), the results of pore distribution measurement using DFT showed that the pore width of rice husk charcoal was mainly concentrated between 0.4 and 0.6 nm, whereas the pore width of the oxygen reduction reaction catalyst was concentrated in two areas, between 0.4 and 0.5 nm and between 0.6 and 0.8 nm, confirming the existence of a hierarchical porous structure. Specifically, in the oxygen reduction reaction catalyst, the proportion of pores with diameters of 0.4 to 0.8 nm (proportion of the number of pores) among pores with diameters of 1.5 nm or less was 82.9%. Furthermore, the average pore diameter of the oxygen reduction reaction catalyst was 0.7 to 0.8 nm, which is consistent with the presence of K in the oxygen reduction reaction electrolyte. + (diameter 0.66 nm), Na + (diameter 0.72 nm), Cl - (0.66 nm diameter), PO4 -3 (diameter 0.67 nm), OH - It was confirmed that the size of the catalyst is large enough to allow the passage of hydrated ions such as those with a diameter of 0.22 nm. This indicates that the oxygen reduction reaction catalyst has superior catalytic properties compared to rice husk charcoal, as it can easily transport these hydrated ions to the active site.

[0045] [X-ray Photoelectron Spectroscopy (XPS) Analysis] The oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH) were analyzed by XPS. The obtained Su1s spectrum, N1s spectrum, and Fe2p spectrum are shown in Figures 7(a) to 7(c), respectively. The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C in the heat treatment process.

[0046] As shown in Figure 7(a), both the oxygen reduction catalyst and rice husk charcoal exhibited two Si peaks between 100 and 200 eV, a C peak between 280 and 290 eV, and an O peak between 526 and 536 eV. Furthermore, the oxygen reduction catalyst exhibited a peak corresponding to N1s at 400 eV, whereas the rice husk charcoal exhibited no such peak. This likely indicates that N was added to the oxygen reduction catalyst by adding urea before the heat treatment process. Similarly, the oxygen reduction catalyst exhibited a peak corresponding to Fe2p at 704 and 736 eV, whereas the rice husk charcoal exhibited almost no such peak. This likely indicates that Fe was added to the oxygen reduction catalyst by using pyrite.

[0047] As shown in Figure 7(b), the N1s spectra for both the oxygen reduction catalyst and rice husk charcoal were confirmed to be divided into four spectra: a spectrum with a peak at 398.3 eV corresponding to pyridinic nitrogen (P), a spectrum with a peak at 399.4 eV corresponding to pyrrolic nitrogen (P), a spectrum with a peak at 400.8 eV corresponding to graphitic nitrogen (Graphitic nitrogen), and a spectrum with a peak at 403.6 eV corresponding to oxidized nitrogen (Oxidized nitrogen). Compared to rice husk charcoal, the oxygen reduction catalyst has a higher ratio of pyridinic nitrogen and graphitic nitrogen, which is expected to improve the electron-withdrawing effect of pyridinic nitrogen and improve conductivity due to graphitic nitrogen, resulting in superior catalytic properties. Furthermore, the oxygen reduction catalyst has a lower ratio of pyrrolic nitrogen, which promotes the two-electron transfer pathway, compared to rice husk charcoal, which is thought to activate the oxygen reduction reaction via the four-electron transfer pathway.

[0048] As shown in Figure 7(c), the Fe2p spectrum shows two clear peaks in the oxygen reduction catalyst, whereas no peak was observed in the rice husk charcoal. This result is thought to indicate that the use of pyrite added Fe to the oxygen reduction catalyst. Furthermore, the presence of a satellite peak in the oxygen reduction catalyst suggests that Fe and N interact with each other. Furthermore, the presence of two peaks, Fe2p3 / 2 and Fe2p1 / 2, associated with the Fe-Nx structure was confirmed in the oxygen reduction catalyst. Since the Fe-Nx structure and satellite peaks generate active sites, the oxygen reduction catalyst is thought to have superior catalytic properties compared to rice husk charcoal.

[0049] [X-ray Absorption Fine Structure (XAFS) Analysis] The oxygen reduction reaction catalyst (RH-Si-Fe-NC) was analyzed by X-ray absorption fine structure (XAFS). First, the XAFS spectrum of the Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), as well as the radial distance distribution function obtained by Fourier transform of the EXAFS spectrum, are shown in Figures 8(a) and 8(b), respectively. The oxygen reduction reaction catalyst was produced at a heating temperature of 750°C in the heat treatment process.

[0050] As shown in Figure 8(a), the oxygen reduction reaction catalyst has an absorption edge between FeO and FePc, suggesting that the oxidation state of iron is between +2 and +3. Furthermore, a second peak exists at 7111 eV, which is closer to FePc than Fe2O3, suggesting that the iron is +2, not +3.

[0051] As shown in Figure 8(b), the oxygen reduction catalyst exhibited a broad peak between 1 and 2 Å. This corresponds to the Fe-N pathway at 1.59 Å. A peak was also observed around 2.60 Å. This corresponds to the Fe-O pathway and is thought to indicate the coexistence of Fe-N4 moieties and Fe clusters with surface oxides. Furthermore, the oxygen reduction catalyst did not exhibit an Fe-Fe pathway, suggesting that the Fe atoms are dispersed and bonded to N, C, and other atoms. This suggests that the central iron atom bonds to other atoms, such as N and C, to form an active site similar to FeN4 porphyrin.

[0052] Next, the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at the Si K edge, as well as the radial distance distribution function obtained by Fourier transform of the EXAFS spectrum, are shown in Figures 9(a) and 9(b), respectively.

[0053] As shown in Figure 9(a), it was confirmed that the oxygen reduction reaction catalyst has an absorption edge between SiO2 and pure Si. It was also confirmed that the peak between 1847 eV and 1850 eV of the oxygen reduction reaction catalyst has a size between that of silica (SiO2) and amorphous silica (Amorphous SiO2), and is particularly close to that of amorphous silica.

[0054] As shown in Figure 9(b), the oxygen reduction catalyst exhibited a peak near 1.3 Å. This peak position perfectly coincided with the peak positions of silica and amorphous silica. Furthermore, the oxygen reduction catalyst exhibited a spectral shape, including fine peak positions, in the EXAFS region from 2 to 4 Å that closely resembled the spectral shape of amorphous silica, rather than silica. Furthermore, the oxygen reduction catalyst did not exhibit any peaks corresponding to the peak positions of pure Si, such as a peak near 2 Å. This suggests that the Si in the oxygen reduction catalyst is chemically bonded to other elements, and is present in large amounts, particularly as amorphous silica.

[0055] [Electrocatalytic Activity Under Alkaline Conditions] Linear sweep voltammetry (LSV) was performed to investigate the electrocatalytic activity of oxygen reduction reaction catalysts (RH-Si-Fe-NC) and rice husk charcoal (pristine RH) under alkaline conditions. For LSV, the electrode containing each catalyst served as the working electrode, a reversible hydrogen electrode (RHE) as the reference electrode, and a platinum counter electrode. Measurements were performed using a potentiostat in a three-electrode cell. The scan rate for LSV was 10 mV / s, the electrolyte was 0.1 M KOH solution, and the stirring speed of the electrolyte was 1600 rpm.

[0056] The voltammogram obtained by LSV is shown in FIG. 10(a). 1 / 2 and limiting current density J ｋ The graph of Fig. 10(b) shows the graph, and the Tafel slope is shown in Fig. 10(c). In the figure and below, the oxygen reduction reaction catalysts heated to 850°C in the heat treatment process are referred to as "RH-Si-Fe-NC," those heated to 750°C as "RHN-(@)750°C," and those heated to 950°C as "RHN-(@)950°C." For comparison, measurements were also performed on a commercially available carbon catalyst carrying 40 wt% platinum (hereinafter referred to as "PtC catalyst").

[0057] As shown in Figure 10(a), the onset potential was 1.04 V for the PtC catalyst, while it was almost the same for RH-Si-Fe-NC at 1.02 V, 0.947 V for RHN-750°C, 0.98 V for RHN-950°C, and 0.76 V for rice husk charcoal. From these results, it is thought that increasing the heating temperature in the heat treatment process from 750°C to 850°C promotes the formation of micropores, and further increasing it from 850°C to 950°C causes the micropores to collapse, blocking the active sites.

[0058] As shown in FIG. 10(b), RH-Si-Fe-NC has a half-wave potential E 1 / 2 is 0.863 V, and the limiting current density J ｋis 5.75 mA / cm 2 and 0.861 V and 5.69 mA / cm for the PtC catalyst, respectively. 2 It was also confirmed that the half-wave potential E 1 / 2 and limiting current density J ｋ It was also confirmed that the value of the onset potential showed a similar tendency to that shown in FIG.

[0059] Furthermore, as shown in Figure 10(c), the Tafel slope of RH-Si-Fe-NC was 68.6 mV / dec. -1 and 98.6 mV / dec for the PtC catalyst. -1 It was confirmed that the ORR rate was smaller and faster than that of the RH-Si-Fe-NCs. From this, it is believed that the superior properties of the RH-Si-Fe-NCs shown in Figures 10(a) and 10(b) are due to the fast ORR rate.

[0060] The electron transfer number and hydrogen peroxide yield obtained by LSV are shown in Figure 11(a). To evaluate the applicability of RH-Si-Fe-NCs to fuel cells, durability tests were performed on RH-Si-Fe-NCs and a commercially available PtC catalyst using an oxygen-saturated KOH electrolyte at 0.8 V vs. RHE. Figure 11(b) shows the results. To investigate the presence of Fe-Nx active sites in RH-Si-Fe-NCs, a voltammogram was obtained by adding KCN to the electrolyte during LSV measurement. Figure 11(c) shows the voltammogram obtained by adding KCN to the electrolyte during LSV measurement. This indicates that cyanide (CN) reacts with the active sites in Fe-Nx. - This test applies the idea that, because of competition between O2 and Fe-Nx, the addition of KCN significantly reduces the ORR activity of catalysts containing Fe-Nx active sites.

[0061] As shown in Figure 11(a), the electron transfer numbers were greater than 3.6 for all oxygen reduction reaction catalysts, with an average of 3.9 for RH-Si-Fe-NC. This is nearly equivalent to the electron transfer number of 4 for the commercial PtC catalyst, indicating that the oxygen reduction reaction proceeds via a four-electron transfer pathway. The hydrogen peroxide yields were also found to be less than 15% on average for all oxygen reduction reaction catalysts, with RH-Si-Fe-NC showing an average of less than 5%. This is similar to the hydrogen peroxide yield of the commercial PtC catalyst, which is less than 2%, and is consistent with the high onset potential shown in Figure 10(a).

[0062] Furthermore, as shown in Figure 11(b), in the durability test, after 14 hours, the PtC catalyst lost 25% of its initial current, while the RH-Si-Fe-NC catalyst lost only 8.9%, maintaining 91.1% of its initial current. This is thought to be because the additive elements, such as Si, Fe, and N, are well dispersed and embedded in the carbon structure of the RH-Si-Fe-NC, preventing erosion of the active sites and facilitating faster interfacial charge transfer between the active sites.

[0063] As shown in Figure 11(c), after adding KCN, the onset potential was confirmed to decrease by 153 mV. However, when the KCN was washed away with water and the electrolyte was returned to 0.1 M KOH solution only, the onset potential was confirmed to recover significantly. This is because the addition of KCN - temporarily reacted with Fe and blocked the active site, but CN - This is thought to be because washing away the Fe-Nx active sites restored the initial state of the oxygen reduction reaction, indicating that RH-Si-Fe-NC contains Fe-Nx active sites.

[0064] [Electrocatalytic activity under neutral conditions] Linear sweep voltammetry (LSV) was performed to investigate the electrocatalytic activity under neutral conditions for the oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH). The LSV conditions were the same as those shown in Figures 10 and 11, except that the electrolyte was a 0.1 M PB solution. The voltammogram obtained by LSV is shown in Figure 12(a), and the half-wave potential E 1 / 2 and limiting current density J ｋ The graph of the graph is shown in Fig. 12(b), and the Tafel slope is shown in Fig. 12(c). The oxygen reduction reaction catalyst was heated to a temperature of 850°C in the heat treatment process. For comparison, measurements were also performed on a commercially available PtC catalyst.

[0065] As shown in Figure 12(a), the onset potential was 0.99 V for the PtC catalyst, while it was 0.96 V for the RH-Si-Fe-NC, which was almost the same as the PtC catalyst, and 0.5 V for the rice husk charcoal. This suggests that RH-Si-Fe-NC has excellent electrocatalytic activity under neutral conditions, similar to that under alkaline conditions shown in Figure 10(a). This is thought to be due to the numerous micropores observed by SEM in Figure 2(c) and the numerous defects observed in the Raman spectrum in Figure 5(a), exposing active sites.

[0066] As shown in FIG. 12(b), RH-Si-Fe-NC has a half-wave potential E 1 / 2 is 0.75 V, and the limiting current density J ｋ is 4.72 mA / cm 2 and 0.705 V and 4.6 mA / cm for the PtC catalyst, respectively. 2 As shown in Fig. 12(c), the Tafel slope of RH-Si-Fe-NC was 66.7 mV / dec. -1 and 175.6 mV / dec for the PtC catalyst. -1It was confirmed that the ORR rate was smaller and faster than that of the RH-Si-Fe-NCs. Therefore, the superior properties of the RH-Si-Fe-NCs shown in Figures 12(a) and 12(b) are attributed to the fast ORR rate.

[0067] Figure 13(a) shows the electron transfer number and hydrogen peroxide yield obtained by LSV. To evaluate the applicability of RH-Si-Fe-NCs to fuel cells, durability tests were performed on RH-Si-Fe-NCs and a commercially available PtC catalyst using an oxygen-saturated PB electrolyte at 0.8 V vs. RHE. Figure 13(b) shows the results. To evaluate the methanol (CHOH) resistance, methanol was injected twice into the electrolyte during the durability test. Figure 13(c) shows the results. This was performed because expensive precious metals are susceptible to the effects of methanol, and this is considered an important indicator for their replacement.

[0068] As shown in Figure 13(a), the electron transfer number was approximately 4 for RH-Si-Fe-NC and the commercial PtC catalyst, but was slightly higher for RH-Si-Fe-NC. This indicates that the oxygen reduction reaction proceeds via a four-electron transfer pathway. Furthermore, the hydrogen peroxide yield was 1.52% for RH-Si-Fe-NC, which was lower than the 2.86% for the commercial PtC catalyst. This result is consistent with the higher onset potential shown in Figure 12(a).

[0069] Furthermore, as shown in Figure 13(b), in the durability test, after 14 hours, the PtC catalyst lost 36% of its initial current, while the RH-Si-Fe-NC catalyst lost only 10%, maintaining 90% of its initial current. This is thought to be because the additive elements, such as Si, Fe, and N, are well dispersed and embedded in the carbon structure of the RH-Si-Fe-NC, preventing erosion of the active sites and facilitating faster interfacial charge transfer between the active sites.

[0070] Furthermore, as shown in Figure 13(c), the current of the PtC catalyst dropped sharply by 40% during the first methanol administration, whereas the current of the RH-Si-Fe-NC catalyst remained almost unchanged. Furthermore, the current of the PtC catalyst also dropped sharply during the second methanol administration, although not as sharply as the first administration. However, the current of the RH-Si-Fe-NC catalyst showed a slight, temporary drop, but quickly recovered. These results suggest that the RH-Si-Fe-NC catalyst has excellent methanol resistance and is suitable as a catalyst for fuel cells.

[0071] From the results shown in FIGS. 12 and 13, it was confirmed that the oxygen reduction reaction catalyst has relatively excellent properties not only under alkaline conditions but also under neutral conditions.

[0072] [Battery Performance under Neutral Conditions] To investigate the electrocatalytic activity of oxygen reduction reaction catalysts (RH-Si-Fe-NC) and PtC catalysts under neutral conditions, various measurements were performed using an alkaline zinc-air battery. The zinc-air battery used 6 M KOH and 0.2 M Zn(Ac)2 electrolytes, an electrode containing either RH-Si-Fe-NC or PtC catalyst as the positive electrode (cathode), and zinc as the negative electrode. The open-circuit voltage, constant-current discharge voltage, and discharge polarization curves are shown in Figures 14(a)–(c), respectively.

[0073] As shown in Figure 14(a), the closed circuit voltage (OCV) of the oxygen reduction reaction catalyst was 1.49 V, which was higher than that of the PtC catalyst (1.42 V). As shown in Figure 14(b), the OCV of the oxygen reduction reaction catalyst was 1.49 V, which was higher than that of the PtC catalyst (1.42 V). 2 to 100 mA / cm 2 Then, increase stepwise to 5 mA / cm 2 When the current density was returned to 5 mA / cm, it was confirmed that the oxygen reduction reaction catalyst had a higher voltage than the PtC catalyst at each current density. 2It was also confirmed that the voltage drop when the battery was returned to normal was very small. These results indicate that the oxygen reduction reaction catalyst not only has an excellent discharge rate, but also long-term durability and reversibility.

[0074] As shown in FIG. 14(c), the oxygen reduction reaction catalyst has a power density of 103 mW / cm 2 and 78 mW / cm 2 It was confirmed that the kinetic energy of the oxygen reduction reaction catalyst is 30% or more higher than that of the PtC catalyst. The results shown in Figure 14 confirm that excellent battery performance can be obtained by using the oxygen reduction reaction catalyst. This is thought to be due to the large number of highly dispersed active sites formed in the oxygen reduction reaction catalyst.

[0075] [Electrocatalytic activity under acidic conditions] Linear sweep voltammetry (LSV) was performed to investigate the electrocatalytic activity under acidic conditions for the oxygen reduction reaction catalyst (RH-Si-Fe-NC) and rice husk charcoal (Pristine RH). The oxygen reduction reaction catalyst was heated to a temperature of 850°C during the heat treatment process. The LSV conditions were the same as those shown in Figures 10 and 11, except that the electrolyte was a 0.1 M HClO solution. The voltammogram obtained by LSV is shown in Figure 15(a), and the half-wave potential E 1 / 2 and limiting current density J ｋ The results are shown in Figure 15(b). For comparison, measurements were also performed on a commercially available PtC catalyst. To evaluate the applicability of RH-Si-Fe-NC to fuel cells, durability tests were performed on RH-Si-Fe-NC and a commercially available PtC catalyst using a 0.1 M HClO electrolyte at 0.8 V vs. RHE. The results are shown in Figure 15(c).

[0076] As shown in Figure 15(a), the onset potential was 0.9 V for the PtC catalyst, while it was almost the same for RH-Si-Fe-NC (0.86 V) and 0.7 V for rice husk charcoal. This suggests that RH-Si-Fe-NC has excellent electrocatalytic activity under acidic conditions, similar to the alkaline conditions shown in Figure 10(a) and the neutral conditions shown in Figure 12(a). This is likely due to the numerous micropores observed by SEM in Figure 2(c) and the numerous defects observed in the Raman spectrum in Figure 5(a), exposing active sites.

[0077] As shown in FIG. 15(b), RH-Si-Fe-NC has a half-wave potential E 1 / 2 is 0.68 V, and the limiting current density J ｋ is 5.4 mA / cm 2 and 0.71 V and 5.5 mA / cm for the PtC catalyst, respectively. 2 It was confirmed that the results were almost equivalent to and superior to those of the previous results.

[0078] Furthermore, as shown in Figure 15(c), in the durability test, after 14 hours, the PtC catalyst lost 28% of its initial current, while the RH-Si-Fe-NC catalyst lost only 16%, maintaining 84% of its initial current. This is thought to be because the additive elements, such as Si, Fe, and N, are well dispersed and embedded in the carbon structure of the RH-Si-Fe-NC, preventing erosion of the active sites and facilitating faster interfacial charge transfer between the active sites.

[0079] From the results shown in FIG. 15, it was confirmed that the oxygen reduction reaction catalyst has relatively excellent properties not only under alkaline or neutral conditions but also under acidic conditions.

[0080] An oxygen reduction reaction catalyst was produced by the method for producing an oxygen reduction reaction catalyst according to an embodiment of the present invention, as shown in Figure 1. In the production process, the heating temperature in the heat treatment step was set to 750°C, and the other conditions were the same as those in Example 1 to produce an oxygen reduction reaction catalyst (RH-Si-Fe-NC, or hereinafter also referred to as "RH-Fe-N"). In addition, rice husk charcoal (Pristine RH) was also produced as a comparative example under the same conditions as in Example 1.

[0081] Linear sweep voltammetry (LSV) was performed to investigate the electrocatalytic activity under various conditions using the manufactured oxygen reduction reaction catalyst (RH-Fe-N), rice husk charcoal (pristine RH), and, as a comparative example, a commercially available carbon catalyst carrying 40 wt.% platinum (PtC catalyst, or hereinafter also referred to as "PtC 40 wt.%"), as in Example 1. In LSV, the electrode containing each catalyst was used as the working electrode, a reversible hydrogen electrode (RHE) as the reference electrode, and platinum as the counter electrode, and measurements were performed using a potentiostat in a three-electrode cell.

[0082] LSV was performed under three conditions: alkaline, neutral, and acidic. The electrolyte was a 0.1 M KOH solution for alkaline conditions, a 0.1 M PB solution for neutral conditions, and a 0.1 M H2SO4 solution for acidic conditions. The scan rate and stirring speed for the LSV were 10 mV / s and 1600 rpm, respectively.

[0083] In addition, five types of electrodes containing the following catalysts were used as the working electrodes in the LSV: oxygen reduction reaction catalyst (RH-Fe-N) only; a mixture of oxygen reduction reaction catalyst (RH-Fe-N) and commercially available carbon black in a 4:1 ratio (mass ratio) (hereinafter referred to as "RH-Fe-N-CB"); rice husk charcoal (Pristine RH) only; a mixture of rice husk charcoal (Pristine RH) and commercially available carbon black in a 4:1 ratio (mass ratio) (hereinafter referred to as "Pristine RH CB"); and a carbon catalyst (PtC 40 wt.%) only.

[0084] The voltammograms obtained by LSV under each condition are shown in Figures 16(a) to 16(c), and the Tafel slopes under each condition are shown in Figure 17. Note that Figure 17 shows the results for three types of electrodes: RH-Fe-N, RH-Fe-N-CB, and PtC 40 wt.%.

[0085] As shown in FIG. 16(a), under alkaline conditions, the current was 0.01 mA / cm 2The onset potentials at 0.99 V for RH-Fe-N, 1.00 V for 40 wt.% PtC, and 0.76 V for Pristine RH were measured. The half-wave potentials were 0.80 V for RH-Fe-N, 0.84 V for 40 wt.% PtC, and 0.65 V for Pristine RH. These results show the same trends as those shown in Figures 10(a) and 10(b). Furthermore, for the RH-Fe-N-CB and Pristine RH-CB catalysts containing carbon black, the onset potentials were almost the same as those without carbon black, whereas the current density was significantly improved in the diffusion region of the LSV plot.

[0086] As shown in Figure 16(b), under neutral conditions, the onset potential was 0.90 V for RH-Fe-N, 1.00 V for 40 wt.% PtC, and 0.76 V for Pristine RH. The half-wave potentials were 0.73 V for RH-Fe-N, 0.78 V for 40 wt.% PtC, and 0.40 V for Pristine RH. These results show the same trends as Figures 12(a) and 12(b). Furthermore, for the RH-Fe-N-CB and Pristine RH-CB catalysts, in which carbon black was added, the onset potential was almost the same as that of the catalysts without carbon black. However, the current density was improved in the diffusion region of the LSV plot, although not as much as under alkaline conditions.

[0087] As shown in Figure 16(c), under acidic conditions, the onset potential was 0.91 V for RH-Fe-N, 0.92 V for 40 wt.% PtC, and 0.80 V for Pristine RH. The half-wave potentials were 0.68 V for RH-Fe-N, 0.71 V for 40 wt.% PtC, and 0.40 V for Pristine RH. These results show the same trends as Figures 15(a) and 15(b). Furthermore, for the RH-Fe-N-CB and Pristine RH-CB catalysts containing carbon black, the onset potential was almost the same as that of the catalysts without carbon black. However, the current density was significantly improved in the diffusion region of the LSV plot, as in the case of alkaline conditions.

[0088] As shown in FIG. 17, the Tafel slope under alkaline conditions was 59.7 mV / dec for RH-Fe-N-CB. -1 and 62.5 mV / dec for PtC 40 wt.%. -1 The Tafel slope under neutral conditions was 48.1 mV / dec for RH-Fe-N-CB. -1 and 96.1 mV / dec for PtC 40wt.%. -1 The Tafel slope under acidic conditions was 51.8 mV / dec for RH-Fe-N-CB, which was smaller than that under alkaline conditions, confirming that RH-Fe-N-CB had a faster ORR rate. -1 and 50.5 mV / dec for PtC 40 wt.%. -1 It was confirmed that the ORR rate was almost the same as that of the conventional method and that the ORR rate was fast.

[0089] 16(a)-(c) and 17 show that, under alkaline, neutral, and acidic conditions, the ORR activity of RH-Fe-N and RH-Fe-N-CB is significantly higher than that of Pristine RH and Pristine RH CB and comparable to that of 40 wt.% PtC. Furthermore, under alkaline, neutral, and acidic conditions, the addition of carbon black to the catalyst improves current density. This suggests that the carbon black improves the diffusion of oxygen ions and protons, resulting in even better ORR activity.

[0090] Three types of electrodes, RH-Fe-N, RH-Fe-N-CB, and 40 wt.% PtC, were subjected to durability tests at 0.8 V vs. RHE for 14 hours in oxygen-saturated PB electrolyte (neutral condition) and 0.1 M H2SO4 electrolyte (acidic condition). The results are shown in Figure 18(a) and (b).

[0091] As shown in Figure 18(a), under neutral conditions, the PtC catalyst experienced a 36% loss, while the RH-Fe-N and RH-Fe-N-CB catalysts experienced only a 25% and 20% loss, respectively. The RH-Fe-N-CB catalysts maintained 80% of their initial current. Also, as shown in Figure 18(b), under acidic conditions, the PtC catalyst experienced a 28% loss, while the RH-Fe-N and RH-Fe-N-CB catalysts experienced only a 16% and 11% loss, respectively. The RH-Fe-N-CB catalysts maintained 89% of their initial current. These results suggest that the addition of carbon black improved the diffusion of oxygen ions and protons, thereby improving durability.

Claims

1. A method for producing an oxygen reduction reaction catalyst, comprising: a preliminary carbonization step in which a raw material containing rice husks is placed in a solution containing iron and sulfuric acid and subjected to hydrothermal carbonization to obtain rice husk charcoal; a heat treatment step in which sodium chloride and urea are added to the rice husk charcoal obtained in the preliminary carbonization step, mixed, and then heated in a nitrogen atmosphere for heat treatment; a neutralization step in which the product obtained in the heat treatment step is neutralized by treating it with an acid; and a secondary carbonization step in which the product obtained in the neutralization step is heated in a nitrogen atmosphere for heat treatment to obtain an oxygen reduction reaction catalyst.

2. The method for producing an oxygen reduction reaction catalyst according to claim 1, characterized in that it comprises a pretreatment step of obtaining the solution containing iron and sulfuric acid by powdering pyrite and subjecting it to hydrothermal treatment, and in the preliminary carbonization step, the solution obtained in the pretreatment step is used for the hydrothermal carbonization treatment.

3. The method for producing an oxygen reduction reaction catalyst according to claim 1, characterized in that the preliminary carbonization step comprises hydrothermal carbonization treatment under temperature and pressure conditions of 150°C to 300°C and 1 MPa to 10 MPa for 1 to 8 hours.

4. The method for producing an oxygen reduction reaction catalyst according to claim 1, wherein the heat treatment step is performed in a nitrogen atmosphere at 700 to 1000°C for 1 to 3 hours.

5. The method for producing an oxygen reduction reaction catalyst according to claim 1, wherein the heat treatment step is performed in a nitrogen atmosphere at 700 to 900°C for 1 to 3 hours.

6. The method for producing an oxygen reduction reaction catalyst according to claim 1, wherein the secondary carbonization step comprises heat treatment in a nitrogen atmosphere at 750 to 950°C for 0.5 to 2 hours.

7. The method for producing an oxygen reduction reaction catalyst according to claim 2, wherein the pretreatment step comprises hydrothermal treatment under temperature and pressure conditions of 150°C to 200°C and 0.8 MPa to 5 MPa for 0.5 to 2 hours.

8. An oxygen reduction reaction catalyst characterized by being porous, containing C, N, Fe, and Si, and having the shape of uniform, wrinkled carbon nanoflakes.

9. The oxygen reduction reaction catalyst according to claim 8, wherein the N and the Fe are added to rice husk charcoal.

10. The oxygen reduction reaction catalyst according to claim 8, which has amorphous silica and a site where the Fe is bonded to the N.

11. The oxygen reduction reaction catalyst according to claim 8, wherein pores having a diameter of 0.4 nm to 0.8 nm account for 80% or more of the pores having a diameter of 1.5 nm or less.

12. The oxygen reduction reaction catalyst according to claim 11, characterized in that the pore distribution of pores having a diameter of 1.5 nm or less has multiple peak positions.

13. The oxygen reduction reaction catalyst according to claim 11, characterized in that, among pores having a diameter of 1.5 nm or less, the pore size distribution has peak positions at least in two ranges: a diameter range of 0.4 nm to 0.5 nm and a diameter range of 0.6 nm to 0.8 nm.

14. A cathode for a fuel cell, comprising the oxygen reduction reaction catalyst according to any one of claims 8 to 13.

15. The cathode for a fuel cell according to claim 14, which contains a mixture of the oxygen reduction reaction catalyst and a conductive additive.

16. The cathode for a fuel cell according to claim 15, wherein the conductive additive is carbon black.

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