KR102385067B1 - Co catalyst for oxygen evolution reaction and The manufacturing method for the same - Google Patents

Abstract

The present invention, on a reduced graphene oxide support, due to the Kirkendall effect, tricobalt tetraoxide (Co ₃ O ₄ ) By having a hollow nanoparticle dispersed structure, electrochemical suitable for water electrolysis It relates to a cobalt catalyst for oxygen evolution reaction having reaction activity and remarkably improved catalyst stability under acidic high potential conditions and a method for producing the same, wherein the cobalt catalyst comprises cobalt oxide including tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles and Because of the strong interaction between the reduced graphene oxide as the support, it is firmly fixed even under severe oxidizing conditions, so it is excellent in stability and can increase the performance of the water electrolysis electrode, the water electrolysis membrane electrode assembly and the water electrolysis cell to which the catalyst is applied. there is.

Description

Cobalt catalyst for oxygen evolution reaction and the manufacturing method for the same

The present invention relates to a cobalt catalyst for an oxygen evolution reaction and a preparation method.

Because hydrogen energy is environmentally friendly, recyclable, and has a high energy density, it can be used in many applications as an energy carrier. A water electrolyzer is one of the promising methods for producing pure hydrogen. In particular, a polymer electrolyte membrane water electrolysis (PEMWE) using a polymer electrolyte membrane has the advantage of high hydrogen production rate and no electrolyte loss.

However, the slow oxygen evolution reaction (OER) at the anode is still an obstacle to the commercialization of PEMWE, and in order to overcome this, the development of a catalyst for PEMWE having excellent reaction rate and activity is required. OER catalysts require high activity and durability under high-potential operating conditions of acidic electrolytes, and in general, pure metal catalysts and oxide catalysts based on iridium and ruthenium are used. These catalyst materials have been developed in various forms such as bulk oxides, nanodendrites, nanoporous structures, metal/metal oxides, and film structures. Nevertheless, it has been difficult to meet high catalytic activity and durability while reducing the amount of metal.

On the other hand, research to use cheaper transition metals such as cobalt (Co) and nickel (Ni) as OER catalyst materials instead of expensive noble metals is ongoing. Due to the problem of passivation at the anodic potential, there is a need to develop a very stable cobalt (oxide) catalyst under high anodic potential and under severe acidic conditions.

Accordingly, the present inventors on the reduced graphene oxide support, through the Kirkendall effect, cobalt trioxide (Co ₃ O ₄ ) When preparing a cobalt catalyst for oxygen generation reaction in which hollow nanoparticles are dispersed, acid We found that it is very suitable for electrochemical reactions that require high potential conditions, and we have completed it.

In order to solve the problems of the prior art as described above, the present invention is a spray pyrolysis method, by heat-treating reduced graphene oxide in which cobalt oxide is dispersed to sequentially go through processes such as reduction and oxidation, due to the Kirkendle effect, tricobalt tetraoxide (Co ₃ O ₄ ) Cobalt for oxygen evolution reaction, in which hollow nanoparticles are dispersed on reduced graphene oxide, have electrochemical reaction activity suitable for water electrolysis, and catalyst stability is significantly improved under acidic high potential conditions An object of the present invention is to provide a catalyst and a method for preparing the same.

In addition, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned are clearly to those of ordinary skill in the art to which the present invention belongs from the description below. can be understood

In the present specification, a) spray pyrolysis of a cobalt precursor solution and graphene oxide to prepare a reduced graphene oxide dispersed in cobalt oxide (Reduced Graphene Oxide); b) primary oxidation or reduction by heat-treating the reduced graphene oxide in which the cobalt oxide is dispersed in the range of 300 to 600 °C; And c) after step b, by oxidation by secondary heat treatment in the range of 250 to 350 ℃, forming a hollow cobalt nanoparticles supported by reduced graphene oxide; It provides a method for preparing a cobalt catalyst for an oxygen evolution reaction comprising a.

In addition, in the present specification, the cobalt precursor solution is cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ 6H ₂ O), and the graphene oxide is prepared by a modified Hummer's method, oxygen evolution reaction A method for preparing a cobalt catalyst for use is provided.

In addition, in the present specification, the cobalt precursor solution has a molar concentration of 0.01 to 0.02 M, and provides a method for preparing a cobalt catalyst for oxygen evolution reaction.

In addition, in the present specification, the primary oxidation step of step b is performed by heat treatment in the range of 300 to 600° C. for 2 to 4 hours under an air or oxygen atmosphere, providing a method for preparing a cobalt catalyst for oxygen evolution reaction.

In addition, in the present specification, the reduction step of step b is carried out by heat treatment in the range of 350 to 450° C. for 2 to 4 hours under 10% hydrogen (H ₂ ) in argon (Ar) atmosphere, cobalt for oxygen evolution reaction A method for preparing a catalyst is provided.

In addition, in the present specification, the secondary oxidation step of step c in the present specification provides a method for preparing a cobalt catalyst for oxygen evolution reaction, which is performed by heat treatment in the range of 250 to 350° C. for 2 to 4 hours under air or oxygen atmosphere.

In the present specification, as a cobalt catalyst prepared according to the method, the cobalt catalyst is cobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles dispersed on a reduced graphene oxide support; and at least one cobalt oxide selected from cobalt (III) oxide (Co ₂ O ₃ ) and cobalt oxide (CoO); It provides a cobalt catalyst for an oxygen evolution reaction, comprising a.

In addition, in the present specification, the average particle diameter of the tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles is in the range of 16.57 ± 5.21 nm, and has a shell of 3 ± 0.5 nm thick, it provides a cobalt catalyst for oxygen evolution reaction.

In addition, in the present specification, the cobalt catalyst, based on the total weight of the catalyst, tricobalt tetraoxide (Co ₃ O ₄ ) containing 80 to 90% by weight of hollow nanoparticles and 10 to 20% by weight of reduced graphene oxide, for oxygen evolution reaction A cobalt catalyst is provided.

In addition, in the present specification, in the cobalt catalyst, Co ³⁺ /Co ²⁺ ratio is 2 or more, it provides a cobalt catalyst for oxygen evolution reaction.

In addition, in the present specification, the cobalt catalyst provides a cobalt catalyst for oxygen evolution, which maintains 80% or more of the initial current after 5,000 dislocation cycles.

In the present specification, it provides an electrode for water electrolysis, including the cobalt catalyst for the oxygen evolution reaction.

In the present specification, it provides a membrane electrode assembly for water electrolysis, including the cobalt catalyst for the oxygen evolution reaction.

In the present specification, a water electrolysis cell comprising the cobalt catalyst for the oxygen evolution reaction is provided.

The cobalt catalyst for oxygen evolution according to the present invention has a structure in which cobalt trioxide (Co ₃ O ₄ ) hollow nanoparticles are dispersed on a reduced graphene oxide support due to the Kirkendall effect As a result, it has electrochemical reaction activity suitable for water electrolysis, and catalyst stability is remarkably improved under acidic high potential conditions.

In addition, the cobalt catalyst for oxygen evolution according to the present invention is firmly fixed even under severe oxidizing conditions due to the strong interaction between cobalt oxide including tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles and reduced graphene oxide as a support. , it is excellent in stability, and the performance of an electrode for water electrolysis, a membrane electrode assembly for water electrolysis, and a water electrolysis cell to which a catalyst is applied can be increased.

1 schematically shows a process for preparing a cobalt catalyst for an oxidation reaction according to an embodiment of the present invention.
2 shows an SEM image of reduced graphene oxide in which cobalt oxide synthesized at various concentrations is dispersed according to an embodiment of the present invention.
3 shows the results of XRD analysis of cobalt catalysts prepared according to one embodiment and comparative examples of the present invention.
4 is a TEM image ((a) to (d)), HAADF-STEM image (e), and elemental mapping ((f) Co, (g) O) of a cobalt catalyst according to an embodiment of the present invention. will be.
5 shows a high-resolution XPS scan for (a) Co _2p , (b) O _1s , (c) C _1s of a cobalt catalyst according to an embodiment of the present invention.
6 is a graph showing cyclic voltammetry curves of cobalt catalysts prepared according to one embodiment and comparative examples of the present invention.
7 shows (a) an LSV curve with iR-correction of a cobalt catalyst prepared according to an embodiment and comparative examples of the present invention, (b) TOF at 1.55 V (vs.RHE), (c) Tafel slope is shown.
Figure 8 is a cobalt catalyst and Ir black prepared according to an embodiment and comparative examples of the present invention (a) potential cycling between 1.3 - 1.65 V in AST, (b) 1.8 V (vs.RHE) after accelerated stress test ) is the normalized current density.
9 is a TEM image ((a) to (d)), HAADF-STEM image (e), elemental mapping ((f) Co, (g) O) is shown.

The terminology used herein is used to describe exemplary embodiments only, and is not intended to limit the invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In this specification, terms such as "comprises", "comprising" or "have" are intended to designate the presence of an embodied feature, step, element, or a combination thereof, but one or more other features or steps; It should be understood that the possibility of the presence or addition of components, or combinations thereof, is not precluded in advance.

Also in the present invention, when it is said that each layer or element is formed "on" or "over" each layer or element, it means that each layer or element is formed directly on each layer or element, or other It means that a layer or element may additionally be formed between each layer, on the object, on the substrate.

Since the invention can have various changes and can have various forms, specific embodiments are illustrated and described in detail below. However, this is not intended to limit the invention to the specific disclosed form, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the invention.

In the present invention, nano means a nanoscale of several nanometers to several tens of micrometers (㎛), specifically, it may mean a size of 10 nm or less, more specifically 5 nm or less . Meanwhile, in the present invention, the term “rGO” used throughout the specification and claims should be understood to mean reduced graphene oxide, and the term “cobalt oxide” is formed by reacting metal cobalt with oxygen, and has a wide range It should be understood to include all of cobalt tetraoxide (Co ₃ O ₄ ), cobalt (III) oxide (Co ₂ O ₃ ), and cobalt oxide (CoO).

Hereinafter, a method for producing a cobalt catalyst for an oxygen evolution reaction according to a specific embodiment of the present invention, a cobalt catalyst for an oxygen evolution reaction prepared accordingly, an electrode for water electrolysis comprising the catalyst, a membrane electrode assembly for water electrolysis, and a water electrolysis cell Explain.

Cobalt catalyst for oxygen evolution reaction and manufacturing method

According to an embodiment of the present invention, a method for preparing a cobalt catalyst for oxygen evolution reaction comprises: a) spraying and pyrolyzing a cobalt precursor solution and graphene oxide to prepare a reduced graphene oxide dispersed with cobalt oxide (Reduced Graphene Oxide); b) primary oxidation or reduction by heat-treating the reduced graphene oxide in which the cobalt oxide is dispersed in the range of 300 to 600 °C; And c) after step b, by secondary oxidation by heat treatment in the range of 250 to 350 ℃, forming a hollow cobalt nanoparticles supported by reduced graphene oxide; may include (FIG. 1).

First, a cobalt precursor solution and graphene oxide are spray pyrolyzed to prepare a reduced graphene oxide in which cobalt oxide is dispersed (step a).

Specifically, an oxygen evolution reaction (OER) catalyst is a catalyst that generates oxygen by decomposing water. Catalyst corrosion can be prevented by first decomposing water before the catalyst is corroded by the atmosphere.

Active materials used as catalysts in the conventional oxygen evolution reaction (OER) include ruthenium (Ru), ruthenium oxide (RuO ₂ ), and iridium oxide (IrO ₂ ). As a result, there was a problem in that it was not suitable for mass production of hydrogen, and there were also disadvantages in terms of catalyst activity and stability, such as being eluted because it was vulnerable to the harsh acidic atmosphere that is the operating condition of the water electrolysis cell. Accordingly, a method of using a carbon support to maximize efficiency by dispersing the catalyst in the form of nanoparticles while reducing the catalyst loading has been proposed. Corrosion caused the catalyst to be desorbed from the support, and there was a problem in that the performance rapidly deteriorated.

On the other hand, according to an embodiment of the present invention, cobalt for oxygen generation reaction in which cobalt trioxide (Co ₃ O ₄ ) hollow nanoparticles are dispersed on a reduced graphene oxide support through Kirkendall effect In the case of preparing the catalyst, it has an electrochemical reaction activity suitable for water electrolysis, and also has the advantage of significantly improving catalyst stability under acidic high potential conditions.

The cobalt precursor solution according to an embodiment of the present invention is a precursor for forming hollow cobalt nanoparticles such as tricobalt tetraoxide (Co ₃ O ₄ ) on the surface of reduced graphene oxide as described below, specifically cobalt nitrate hexahydrate ( Co(NO ₃ ) ₂ 6H ₂ O) may be, for example, having a molar concentration of 0.01 to 0.02 M, specifically, having a molar concentration of 0.015 M. Specifically, when the cobalt nitrate hexahydrate has a molar concentration of 0.015 M, a sufficient amount is loaded on the graphene sheet to sufficiently cover the area of the graphene support, and it is densely covered on the surface of the graphene support without agglomeration. Therefore, it becomes possible to increase the catalytically active surface area (FIG. 2).

Graphene Oxides (GO) according to an embodiment of the present invention may serve as a support by providing excellent electrical conductivity and self-supporting ability while strongly interacting with cobalt nanoparticles. It may be prepared by the Hummer's method. As an example, in the above step, graphene oxide may be used in an amount of 1 mg mL ^-1 .

On the other hand, after preparing a spray precursor solution containing the cobalt precursor solution and graphene oxide, a spray pyrolysis reaction may be performed, and the reactor in which the spray pyrolysis is performed is, for example, an ultrasonic atomizer, a tube furnace with a quartz tube (furnace), and may be comprised of a bag filter for powder collection.

Specifically, the droplets may be generated from a precursor solution using an ultrasonic atomizer, and the droplets are delivered to a quartz tube by a nitrogen (N ₂ ) flow as a carrier gas, and the temperature of the tube furnace is maintained at 800° C. The spray pyrolysis may be performed in a state of being Through the above process, reduced graphene oxide in which cobalt oxide is dispersed is prepared.

Next, the reduced graphene oxide in which the cobalt oxide is dispersed is subjected to a heat treatment in a range of 300 to 600° C. for primary oxidation or reduction (step b).

In this step, by thermal reduction of cobalt oxide having an initial cubic structure, particularly cobalt oxide (CoO), oxygen atoms are released from the oxide, which causes the cobalt atoms to shrink into smaller spherical particles. On the other hand, the reduced graphene oxide support prevents the cobalt oxide particles from being agglomerated.

Meanwhile, the primary oxidation step may be performed by heat treatment in the range of 300 to 600° C. for 2 to 4 hours under air or oxygen atmosphere.

On the other hand, the reduction step may be performed by heat treatment in the range of 350 to 450 °C for 2 to 4 hours under 10% hydrogen (H ₂ ) in argon (Ar) atmosphere.

On the other hand, it should be understood that the terms "primary" and "secondary" used throughout the specification and claims of the present invention are arbitrarily assigned an order for differentiation.

Next, after step b, by secondary oxidation by heat treatment in the range of 250 to 350 ° C., to form hollow cobalt particles supported by reduced graphene oxide (step c).

In the step b, a Kirkendall diffusion effect occurs by continuous thermal oxidation after the thermal oxidation or reduction in step b, and the cobalt oxide is converted into hollow particles, thereby forming voids inside the cobalt particles. Specifically, in step b, cobalt ions (Co ³⁺ ) contracted into small spherical particles diffuse outward more rapidly than oxygen ions (O ² - ) ions, which are counterflow ions in the thermal oxidation process, so that different diffusion rates Accordingly, numerous cavities are formed inside the metal particles, they are connected to each other, and they evolve into voids.

On the other hand, the oxidation in the above step may be carried out by heat treatment in the range of 250 to 350 °C for 2 to 4 hours under air or oxygen atmosphere.

The cobalt catalyst for the oxygen evolution reaction prepared according to the method described above is cobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles dispersed on a reduced graphene oxide support; and at least one cobalt oxide selected from cobalt (III) oxide (Co ₂ O ₃ ) and cobalt oxide (CoO); may include.

On the other hand, the average particle diameter of the tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles is in the range of 16.57 ± 5.21 nm, and may have a shell with a thickness of 3 ± 0.5 nm, and the specific surface area in the BET analysis is, for example, 67.9 m ² g ^-1 .

On the other hand, the cobalt catalyst, based on the total weight of the catalyst, tricobalt tetraoxide (Co ₃ O ₄ ) It may be to include 80 to 90% by weight of hollow nanoparticles and 10 to 20% by weight of reduced graphene oxide, the cobalt catalyst In , the Co ³⁺ /Co ²⁺ ratio may be 2 or more.

On the other hand, in the case of a catalyst in which tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles are present on the surface of the reduced graphene oxide support as described above, the double-site oxygen evolution reaction mechanism can be followed as shown in Scheme 1 below, so that oxygen is generated It can be effective in response.

[Scheme 1: Dual-site OER mechanism of tricobalt tetraoxide]

The cobalt catalyst described above can maintain 80% or more of the initial current after 5,000 potential cycles, and because of such electrochemical reaction activity and catalyst stability under oxidation high potential conditions, an electrode for water electrolysis to which the catalyst is applied, a membrane for water electrolysis It is possible to increase the performance of the electrode assembly and the water electrolysis cell.

On the other hand, the cobalt catalyst for the oxygen evolution reaction prepared by the above method can be used for the oxidation electrode of the water electrolyzer, and when the oxidation electrode is configured as described above, a platinum catalyst is used as a commercial metal catalyst for the reduction electrode as an example, Membrane electrodes can be fabricated using a catalyst-coated membrane (CCM) method. In addition, a reducing electrode, an oxide electrode, and a gasket are placed on the electrolyte membrane of the membrane electrode assembly prepared as described above and fastened together with a bipolar plate made of graphite and Ti to manufacture a unit cell for water electrolysis.

Hereinafter, the action and effect of the invention will be described in more detail through specific examples of the invention. However, this is presented as an example of the invention and the scope of the invention is not limited in any way by this.

[Example]

Example 1: Preparation of cobalt catalyst for oxygen evolution reaction (CG-RO production)

Prepare a spray precursor solution with 0.015 M of cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ 6H ₂ O, Sigma Aldrich) and 1 mg mL ⁻¹ of graphene oxide (GO) synthesized by Hummer’s method modified in distilled water Then, droplets are produced in a USP reactor consisting of an ultrasonic nebulizer, a tube furnace with a quartz tube and a bag filter for powder collection, which is then transferred to a quartz tube with a nitrogen (N ₂ ) stream as a carrier gas. , and maintaining the temperature of the tube furnace at 800° C., through carbonization of graphene oxide and decomposition of the cobalt precursor, reduced graphene oxide in which cobalt oxide was dispersed on the surface was prepared (CG-NT).

Next, the CG-NT was heat-treated as a reducing gas at 10% H ₂ in Ar at 400° C. for 3 hours to obtain a reduced powder (CG-R), which was further reduced by heat treatment at 300° C. in air for 3 hours. By forming hollow cobalt particles supported by graphene oxide (rGO) sheets, a cobalt catalyst for oxygen evolution (CG-RO) was obtained.

Comparative Example 1

A cobalt catalyst was prepared in the same manner as in the above example, except that 0.010 M of cobalt nitrate hexahydrate was used.

Comparative Example 2

A cobalt catalyst was prepared in the same manner as in Example, except that 0.020 M of cobalt nitrate hexahydrate was used.

Comparative Example 3 (manufactured by CG-O)

A cobalt catalyst was prepared in the same manner as in the above example, except that the process of obtaining a reduced powder was omitted by heat treatment at 400° C. for 3 hours under 10% H ₂ in Ar as a reducing gas.

Comparative Example 4

A commercial catalyst, Ir black, was prepared.

Example 2: Preparation of Electrode for Water Electrolysis

10 mg of the catalyst prepared in Example 1, 0.06 ml of Nafion (5 wt%) solution, and 0.8 ml of isopropyl alcohol were mixed and dispersed for 30 minutes through a sonicator to prepare a catalyst ink solution for a half-cell of a water electrolyzer . After loading 0.005 ml of the catalyst ink solution prepared through a micropipette on the working electrode, it was dried at room temperature to complete the thin film electrode. Before loading the catalyst ink, the working electrode was washed with alumina solution, acetone, and distilled water.

Example 3: Preparation of membrane electrode assembly for water electrolysis

An anode catalyst ink for water electrolysis was prepared by dispersing 0.023 mg of the catalyst prepared in Example 1, 0.058 mg of Nafion (5 wt%) solution, and 5 ml of isopropyl alcohol through a sonicator for 30 minutes. The prepared catalyst ink was applied to the Nafion 212 polymer electrolyte membrane oxide electrode through an automatic spray device (1.0 mgPt/cm ² ), and a commercial platinum-supported catalyst was applied to the reduction electrode (0.2 mgPt/cm ² ).

Example 4: Preparation of unit cell for water electrolysis

Carbon paper (reducing electrode), Ti paper (oxidizing electrode), and gasket are placed on the electrolyte membrane of the membrane electrode assembly prepared in Example 3, and fastened together with a bipolar plate made of graphite and Ti material to form a unit cell for water electrolysis Cells were prepared.

[Experiment 1: Confirmation of XRD pattern]

XRD analysis was performed on CG-NT, CG-R, CG-RO, and CG-O according to Example 1 and Comparative Example 3. Referring to FIG. 3 , the CG-NTs prepared by USP show high crystallinity and the same pattern of the cubic CoO phase, and an additional small peak near 44° corresponds to the plane 111 of the metallic cubic Co phase (JCPDS No. 15-0806), indicating that cobalt is partially reduced via USP at 800°C. The reduction of CG-NT to CG-R is confirmed by the clear peaks of metallic cubic cobalt and metallic hexagonal cobalt (JCPDS No.05-0727), which means that CG-NT is completely reduced under H ₂ in Ar at 400° C. . In the case of CG-O thermally oxidized using air, the initial CoO phase is converted to a Co ₃ O ₄ phase (JCPDS No.76-1802), and a small peak of CoO associated with the (200) plane near 42° is also identified. The thermally reduced CG-RO still shows a CoO peak at 42° after oxidation and has a strong diffraction peak equivalent to that of the Co ₃ O ₄ phase.

[Experiment 2: Morphology Measurement]

Referring to FIG. 4 , in the case of the cobalt catalyst according to Example 1, the cobalt oxide particles are well dispersed on the crumpled graphene sheet without serious agglomeration. On the other hand, CG-RO has clear lattice patterns with intervals of 0.467, 0.247, and 0.23 nm, respectively, for the (111), (311), and (222) planes of the Co ₃ O ₄ crystal, which correspond to the XRD results shown in FIG. Matches well.

On the other hand, the particle size distribution and average particle diameter of the cobalt catalyst according to Example 1 and Comparative Example 3 were estimated from about 100 particles, as can be seen in the TEM image of FIG. 4 , CG-NT, CG The average sizes of cobalt oxide particles in -R and CG-O were 10.90 (±3.91), 7.91 (±2.35) and 11.59 (±3.89) nm, respectively, and tricobalt tetraoxide (Co ₃ O ₄ ) nano In the case of particles, it is 16.57 (±5.21) nm, which is more than twice as large as the CoO particles observed in CG-R, and from these results, the cobalt catalyst (CG-RO) according to Example 1 has voids inside the cobalt particles due to the Kirkendle effect. formation could be confirmed. On the other hand, it was confirmed that these hollow tricobalt tetraoxide spheres had a shell with a thickness of about 3 nm (see FIG. 4 (d)).

Meanwhile, the high-angle annular dark field scanning transmission electron microscope (HAADF) and element mapping images of the CG-RO cobalt catalyst according to Example 1 are presented in FIGS. 3E to 3G . These results confirm that the cobalt particles are uniformly distributed on the graphene surface. In particular, elemental cobalt is mainly distributed in the shell, clearly indicating that it is a hollow structured particle.

Meanwhile, the specific surface area and pore size distribution (PSD) of all CG catalysts were investigated by the N ₂ adsorption/desorption method using a BET analyzer. In BET analysis, the specific surface area of the CG catalyst was 43.2, 49.1 52.2 and 67.9 m ² g ⁻¹ for CG-NT, CG-R, CG-O and CG-RO, respectively. The BET results indicate that the rGO support mainly provides surface area for all CG samples of nearly identical unimodal PSD data and surface area, nevertheless, the catalytic (i.e. cobalt oxide) morphology controlled by other thermal processes. (morphology) increases. After thermal reduction of CG-NTs, the increased specific surface area of CG-Rs may correspond to a decreased particle size with increased oxygen atom release. Conversely, thermal oxidation of CG-NTs is thought to accelerate carbon oxidation in the presence of Co oxide to form more micropores. In particular, the much larger surface area of CG-RO is associated with Kirkendl pores formed by counter diffusion between cobalt and oxygen ions, creating more active sites.

[Experiment 3: High-resolution XPS scan experiment]

The valence electronic states of each element in the CG catalyst were characterized by XPS. The Co _2p spectrum of the cobalt catalyst according to Example 1 was separated and displayed in FIG. The _2p spectra are summarized in Table 1.

The main peaks of both Co _2p3/2 and Co _2p1/2 are located at 780.4 and 796.8 eV, respectively. The two strong satellite peaks observed at 786.3 and 802.5 eV confirm the formation of the CoO phase, since the CoO and Co ₃ O ₄ phases are distinguished by the shape and intensity of the satellite peaks.

The Co _2p3/2 main peaks can be separated at 779.4 and 781.5 eV, corresponding to metallic Co and Co ²⁺ , respectively. Similarly, the Co _2p1/2 main peak has discrete peaks representing metallic Co at 794.6 eV and Co ²⁺ at 797.3 eV. The two prominent peaks, Co _2p3/2 and Co _2p1/2 , display a spin-orbital splitting energy of 15.2 eV with two weak satellite peaks, and the surfaces of CG-R, CG-O and CG-RO are of Co ₃ It indicates that it has an O ₄ phase. However, according to the separated peaks, CG-R represents the cobalt oxide phase and not the metallic cobalt identified using XRD and TEM. This may be due to surface oxidation of metallic cobalt in the atmosphere.

The separated Co _2p3/2 peaks observed in FIGS. 4a and S6b-c located at 780.2 and 781.3 eV may be attributed to Co ³⁺ and Co ²⁺ , respectively. Also, the last two peaks of Co _2p1/2 are centered at 795.3 and 797.3 eV assigned to Co ³⁺ and Co ²⁺ , respectively. Therefore, the relative number of each ion can be calculated using the individual peak areas of the Co ³⁺ and Co ²⁺ ions. The molar ratio of Co ³⁺ /Co ²⁺ of CG-RO is about 1.7 times higher than that of CG-O.

Meanwhile, the O _1s spectrum of all samples can be divided into three peaks. Oxygen species (OL), oxygen vacancies (OV) and chemisorbed oxygen (OC) of the lattice of the cobalt catalyst according to Example 1. The separated peaks and their information are shown in FIG. 5 (b), and the O _1s spectra of CG-NT, CG-R, CG-O and CG-RO according to Example 1 and Comparative Example 3 are also shown in Table 2 summarized.

Peaks of 530, 531.4 and 532.4 eV are observed for all samples. The OV/OL ratios of CG-NT, CG-O, and CG-RO were similar at 0.73-0.75, but the ratio of CG-R was much higher, indicating that more oxygen vacancies were formed during thermal reduction.

On the other hand, the C _1s peak is separated into three peaks centered at 284.5, 286, and 3 eV, which are related to CC, COC and OC=O, respectively (Fig. 5c, and Table 3). All CG complexes show strong CC binding peaks and broad COC and OC=O peaks, indicating that GO was sufficiently converted to rGO via USP.

[Experiment 4: Cyclic Voltammetry Measurement]

CV was performed to confirm the surface reaction of the CG catalyst. The catalyst was scanned from 0 to 1.5V (vs.RHE) at a scan rate of 5mVs ⁻¹ in N ₂ -saturated 0.1M HClO ₄ solution and 6 cycles were repeated. As shown in Fig. 6 (a) and (b), a prominent oxidation peak is generally observed at 1.1 - 1.2 V (vs.RHE), which corresponds to a reaction in which Co ²⁺ ions are oxidized to Co ³⁺ ions. . It is believed that CG-NT contains more Co ²⁺ ions than CG-R because CG-NT has a larger peak associated with the oxidation of Co ²⁺ to Co ³⁺ . However, the Co ²⁺ /Co ³⁺ peaks for both CG-NT and CG-R disappear after the first dislocation cycle, indicating that the surfaces of both complexes are unstable in acidic electrolytes.

In addition, since the CoO phase of CG-NT is generally less stable than Co ₃ O ₄ and the Co ₃ O ₄ phase is only found on the surface of CG-R, CG-NT and CG-R stably transport Co ³⁺ ions. It seems impossible. Conversely, as shown in Figs. 6 (c) and (d), small peaks at 1.39 - 1.42 V (vs.RHE) for CG-O and 1.27 - 1.37 V (vs. RHE) for CG-RO are repeated repeatedly. appear. It has been reported that Co ³⁺ ions can continuously initiate the OER process through a redox reaction between Co ³⁺ ions and Co ⁴⁺ ions.

Therefore, this result ensures that both CG-O and CG-RO are more active against OER due to the appearance of the Co ³⁺ /Co ⁴⁺ oxidation peak. More importantly, a relatively low oxidation onset potential is found in CG-RO showing a clear reduction peak from Co ⁴⁺ to Co ³⁺ , which indicates that i) CG-RO initiates OER at a low overpotential, and ii) CG-O It is shown to provide a more reversible interconversion between Co ³⁺ and Co ⁴⁺ . It is well known that the double-layer capacitance (Cdl) of metal oxides can be used to evaluate the electrochemically active surface area. CV curves at various scan rates (5 - 25 mVs ^-1 ) for all complexes are obtained in the non-Faraday potential range of 0.75 - 0.95 V (vs.RHE).

The Cdl value was estimated using the anode current at 0.85V (vs.RHE). A plot of the anode current versus the scan rate was measured, and Cdl was determined at each slope. The roughness coefficient (Rf) was obtained by dividing Cdl by Cs representing the capacitance of the double layer with a smooth surface, and the Cs value was used as 60 μF cm ⁻² . Finally, ECSA was estimated from Rf and geometrical area (S=0.196 cm ² ). The values of Cdl, Rf and ECSA are summarized in Table 4. As listed in Table 4, the ECSA of the CG complex increases in the order of CG-R<CG-O=CG-RO<CG-NT.

[Experiment 5: Electrochemical OER activity evaluation]

To evaluate the electrochemical OER activity of CG catalysts, LSV was performed using RDE at 1,600 rpm and 10 mVs ⁻¹ in N ₂ saturated 0.1M HClO ₄ . Then, to observe the kinetic contribution of the CG complex to the polarization behavior, the experimental LSV data were iR-corrected and presented in Fig. 7(a).

As can be seen in Fig. 7(a), CG-NT and CG-R have relatively low OER activity and cannot achieve 10 mAcm ^-2 at the applied potential. However, the OER performance of CG-O is a much higher onset potential than that of CG-NT and CG-R, requiring 412 mV to reach 10 mAcm ^-2 , which indicates that USP followed by oxidation significantly enhances OER activity, whereas , meaning that the additional thermal reduction rather reduces the OER performance.

In particular, CG-RO subjected to further reduction and subsequent oxidation exhibits a further reduced overpotential (ie, η = 353 mV at 10 mAcm ^-2 ). For comparative purposes, the conversion frequency (TOF) of OER at 1.55 V (vs.RHE), representing the number of moles of oxygen released per second at a unit active site, was calculated using the following equation, and is shown in Fig. 7(b). did

TOF = jA/4Fn

where j is the current density obtained at 1.55 V (vs.RHE), A is the geometric area of the catalyst coated, F is the Faraday constant (= 96,485 Cmol ⁻¹ ) and n is the number of moles of catalyst metal (i.e., Co). As the calculated TOF for the CG catalyst increases exponentially through different thermal processes, it produces molecular oxygen much faster because the intrinsic activity of CG-RO (catalytically active sites available for OER) is much higher than that of other CG catalysts. indicates

In addition, considering the overpotential (η) obtained and estimated at 10 mAcm ^-2 from the literature on a non-noble metal catalyst in an acidic medium with a current density of 1.6V (vs.RHE), according to an embodiment of the present invention, The prepared CG-RO catalyst showed higher performance and is considered as a promising OER catalyst due to the easy green synthesis method with high activity.

On the other hand, since the electrochemical reaction mainly occurs at the interface between the surface of the catalyst and the electrolyte, it is widely known that ECSA is a critical parameter that determines the activity of the electrocatalyst. However, the ECSA trends of the synthesized CG complexes are not consistent with those of OER activity, suggesting that the surface area available for charge transfer to the electrolyte is partially used for OER.

In particular, CG-NTs with higher ECSA do not catalyze OER efficiently, whereas CG-ROs with the same ECSA as CG-O show relatively high OER selectivity due to the more active species with higher intrinsic activity. . That is, OER is one of the other Faradic and non-Faradian reactions that plausibly occur on electroactive surfaces; Thus, other catalytic properties such as surface composition, particle morphology, and interaction with the catalyst support can also affect OER activity.

On the other hand, the kinetic-controlled Tafel slope (b _kin ), which is generally obtained from the current-potential relationship, represents the rate-determining step (RDS) during the electrochemical process at the interface between the electrode and the electrolyte. In particular, for an OER comprising four bound proton-electron transfer steps, if the first electron transfer reaction associated with the formation of an adsorbed hydroxide on the metal surface (M-OH*) is RDS, then b _kin is theoretically 120 mVdec ^-1 , and b _kin becomes 60 mVdec ^-1 when RDS is the second electron transfer reaction related to the formation of MO*. On the other hand, as b _kin is smaller, the overvoltage for increasing the current density becomes smaller, so that even better OER catalyst performance can be exhibited.

Similarly, when the third or fourth electron transfer step (M-OOH* or M) is RDS, the ideal b _kin is 30 - 40 or 15 - 20 mVdec ^-1 according to the various OER mechanisms proposed, respectively. Thus, good OER catalysts have smaller b _kin , indicating that RDS is later found in the electron transport step. The RDS of the CG complex can be inferred using the experimentally determined b _kin in Fig. 7(c). In particular, the b _kin for CG-R is 2.25 times higher than that of the first electron transfer process, indicating that CG-R is virtually inactive for OER since it must occur immediately after molecular oxygen liberation.

CG-NT is much lower than the b _kin of CG-R, but still higher than 120 mVdec ^-1 . Nevertheless, this suggests that Co ²⁺ ions may serve only marginally as active sites of OER. On the other hand, from the b _kin values for CG-O and CG-RO, the hypothesis that the presence of Co ³⁺ ions on Co ₃ O ₄ reduces the energy barrier to the first water dissociation (corresponding to b _kin = 120 mVdec ^-1 ). After deprotonation of hydroxide ions adsorbed to Co ³⁺ ions or with deprotonation, thermodynamically unfavorable secondary water dissociation can become RDS depending on the concentration of Co ³⁺ ions.

[Experiment 6: Stability Test]

In general, since cobalt (oxide) is easily soluble in acidic environments and passivated at high anodic potential, it is necessary to ensure the long-term stability of the CG catalyst prepared in this study. Therefore, CG-RO with higher OER activity was investigated under AST constituting a potential cycling between 1.3 - 1.65 V(vs.RHE) in N ₂ saturated 0.1 M HClO ₄ with a scan rate of 50 mVs ^-1 .

Polarization curves for OER were measured every 1,000 potential cycles using LSV, and each current obtained at 1.8 V (vs.RHE) in LSV during AST was normalized to the initial value before AST. Referring to FIG. 8 , CG-RO maintained about 80% of the initial current after 5,000 dislocation cycles, whereas commercial Ir black was severely degraded to maintain about 1% of OER activity. These results suggest that CG-RO is very stable under high anode potential and harsh acidic conditions.

Meanwhile, to further investigate the degradation of the CG-RO catalyst after AST, TEM and EDS analyzes were performed on CG-RO with 5,000 dislocation cycles. As can be seen in Figure 9, the average diameter of Co ₃ O ₄ particles in CG-RO slightly increased from 16.57 (± 5.21) to 18.04 (± 6.83) nm in the rGO sheet without significant agglomeration.

In particular, the crumpled shape of the rGO sheet and the hollow structure of the Co ₃ O ₄ particles were well maintained during AST. Post-mortem TEM and EDS analysis results showed that i) cobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles and rGO sheets were robust at low pH and high oxidation potential, and ii) cobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles were catalytic particles. The strong interaction between the rGO and the support suggests that they are firmly anchored to the rGO sheet under vigorous oxidizing conditions.

In the foregoing, specific embodiments of the present invention have been described and illustrated, but it is common knowledge in the art that the present invention is not limited to the described embodiments, and that various modifications and variations can be made without departing from the spirit and scope of the present invention. It is self-evident to those who have Accordingly, such modifications or variations should not be individually understood from the spirit or point of view of the present invention, and the modified embodiments should belong to the claims of the present invention.

Claims (14)

a) preparing a reduced graphene oxide in which cobalt oxide is dispersed by spray pyrolysis of a cobalt precursor solution and graphene oxide having a molar concentration of 0.015 M;
b) reducing the reduced graphene oxide in which the cobalt oxide is dispersed by heat treatment for 2 to 4 hours in the range of 350 to 450° C. under 10% hydrogen (H ₂ ) atmosphere in argon (Ar); and
c) after step b, by oxidizing by heat treatment in the range of 250 to 350 ° C. for 2 to 4 hours under air or oxygen atmosphere, thereby forming cobalt hollow nanoparticles supported by reduced graphene oxide; A method for preparing a cobalt catalyst for PEM water electrolysis oxygen evolution reaction comprising a. The method of claim 1,
The cobalt precursor solution is cobalt nitrate hexahydrate (Co(NO ₃ ) ₂ 6H ₂ O), and the graphene oxide is prepared by a modified Hummer's method, PEM water electrolysis for oxygen evolution reaction A method for preparing a cobalt catalyst. delete delete delete delete A cobalt catalyst prepared according to the method of claim 1, comprising:
The cobalt catalyst is, cobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles dispersed on a reduced graphene oxide support; and
at least one cobalt oxide selected from cobalt (III) oxide (Co ₂ O ₃ ) and cobalt oxide (CoO); A cobalt catalyst for PEM water electrolysis oxygen evolution reaction comprising a. 8. The method of claim 7,
The average particle diameter of the tricobalt tetraoxide (Co ₃ O ₄ ) hollow nanoparticles is in the range of 16.57 ± 5.21 nm, and has a shell of 3 ± 0.5 nm thick, PEM water electrolysis cobalt catalyst for oxygen evolution reaction. 8. The method of claim 7,
The cobalt catalyst is, based on the total weight of the catalyst, cobalt trioxide (Co ₃ O ₄ ) containing 80 to 90% by weight of hollow nanoparticles and 10 to 20% by weight of reduced graphene oxide, PEM water electrolysis cobalt catalyst for oxygen evolution reaction . 8. The method of claim 7,
In the cobalt catalyst, Co ³⁺ /Co ²⁺ ratio is 2 or more, PEM cobalt catalyst for water electrolysis oxygen evolution. 8. The method of claim 7,
The cobalt catalyst maintains 80% or more of the initial current after 5,000 dislocation cycles, PEM water electrolysis cobalt catalyst for oxygen evolution. A PEM water electrolysis electrode comprising the cobalt catalyst for the oxygen evolution reaction of claim 7. A membrane electrode assembly for PEM water electrolysis, comprising the cobalt catalyst for the oxygen evolution reaction of claim 7. A PEM water electrolysis cell comprising the cobalt catalyst for the oxygen evolution reaction of claim 7.

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