KR20240006819A - Catalyst for generating double-function hydrogen and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a dual-functional hydrogen generation catalyst to replace platinum-based catalyst electrodes in water electrolysis HER and a method for manufacturing the same. More specifically, it relates to optimal bonding and adhesion stability between a carbon support electrode and CoS/S-rGO catalyst particles. For this purpose, the catalyst particles can be produced by directly growing them on a carbon support electrode using a hydrothermal synthesis method.
The dual-functional hydrogen generation catalyst according to the present invention reduces crystal defects by introducing S into Co and rGO respectively, facilitates water adsorption on the catalyst, and can produce more hydrogen, resulting in an excellent overpotential value. It has the advantage of being able to proceed with the water decomposition reaction by consuming little energy and having excellent stability and durability as well as high efficiency.

Description

Catalyst for generating double-function hydrogen and manufacturing method thereof}

The present invention relates to a dual-functional HER catalyst with high efficiency and long-term stability for hydrogen evolution reaction (HER) in a water electrolysis system and a method for producing the same.

The development of new renewable energy sources that can simultaneously alleviate the energy crisis caused by fossil fuel depletion and global warming caused by various environmental pollutants is receiving great scientific and industrial attention. Various studies have been conducted on eco-friendly and infinite renewable energy sources to replace fossil fuels, and among these, hydrogen energy is a representative energy carrier.

Hydrogen reacts with oxygen in the air to produce only water without producing any other pollutants. The energy density per mass of hydrogen is 142 kJ/g, which is 4 to 3 times higher than that of gasoline and natural gas. There are various ways to produce hydrogen, but currently the most popular way to produce hydrogen is to extract hydrogen from fossil fuels. However, this method causes environmental problems, so one of the eco-friendly hydrogen production methods is the water electrolysis hydrogen production method.

The water electrolysis method requires simple equipment, can be mass-produced, and can produce high-purity hydrogen. However, because water electrolysis technology has the disadvantage of requiring the use of a considerable amount of electrical energy, the key to the water electrolysis system is to lower the energy required to decompose water using a catalyst. Platinum (Pt), which is considered the best catalyst in the hydrogen generation reaction of water electrolysis systems, has high efficiency due to its low overpotential value and Tafel slope, but its small reserves and high price make it difficult to apply in commercialization processes.

Therefore, it is essential to develop new electrochemical catalysts to replace platinum. In addition, metal sulfide and phosphide-based catalysts that have been recently developed are unstable in polar solvents, causing the active ingredients to elute during the reaction, and their conductivity is lower than that of metals or alloys, so they lack stability. Therefore, catalyst electrodes with high efficiency and long-term stability in water electrolysis systems are difficult to achieve. Development is needed.

Republic of Korea Patent No. 10-2201079 (2021.01.05)

The present invention was created to solve the above problems. It operates stably in both acidic and alkaline electrolytes, and is a dual-function catalyst for HER with high efficiency and long-term stability that can replace platinum catalysts in water electrolysis systems. The purpose is to provide a method for manufacturing the same.

In order to achieve the above object, the present invention includes reduced graphene oxide (rGO) doped with sulfur element (S); and cobalt sulfide (CoS) particles attached to the rGO. It provides a dual-functional catalyst for hydrogen generation comprising a.

In addition, the present invention relates to a base material; and a dual-function electrode for hydrogen generation including the catalyst.

In addition, the present invention includes a first step of dispersing graphene oxide (GO) in ethylene glycol (EG) and preparing a GO dispersion; A second step of preparing S-rGO powder by adding a reducing agent to the GO dispersion and stirring it; A third step of mixing the S-rGO powder with distilled water and stirring it to prepare an S-rGO dispersion solution; A fourth step of preparing a first mixed solution by adding a cobalt precursor and a sulfur precursor to the S-rGO dispersion solution and stirring; A fifth step of preparing a second mixed solution by immersing carbon paper (CP) in the first mixed solution and ultrasonicating it; And a sixth step of sealing the CP and the second mixed solution and collecting an electrode containing CoS particles (CoS/S-rGO) attached to rGO doped with elemental sulfur (S) overgrown through hydrothermal synthesis. Provides a method for manufacturing an electrode including;

The dual-function hydrogen generation catalyst according to the present invention exhibits a low overpotential value even though it does not use noble metals, which means that hydrogen can be produced using little energy at a low price. In addition, the dual-functional hydrogen generation catalyst has excellent stability and durability in acidic and alkaline electrolytes, even through 3,000 repeated HER reactions for 10 hours, and can be used for a long time with high efficiency, so it can be used as a commercial material in the future. It is expected that HER can be applied very economically.

Figure 1 is a mechanism for performing HER in alkaline and acidic electrolytes using an electrode according to an embodiment of the present invention.
Figure 2 shows an X-ray diffraction (XRD) pattern, Raman spectrum, and Fourier transform intrared (FT-IR) spectrum of an electrode according to an embodiment of the present invention.
Figure 3 is a scanning electron microscope (SEM) image and an energy dispersive X-ray spectroscopy (EDS) spectrum of an electrode according to an embodiment of the present invention.
Figure 4 is an SEM image of an electrode according to an embodiment of the present invention observed after 3,000 Cyclic Voltammetry (CV) cycles.
Figure 5 is a high-resolution transmission electron microscope (HRTEM) image of an electrode according to an embodiment of the present invention.
Figure 6 shows the result data of X-ray photoelectron spectroscopy (XPS) of an electrode according to an embodiment of the present invention.
Figure 7 shows the XRD pattern, Raman spectrum, and X-ray photoelectron spectrum of CoS/S-rGO scraped from CP before and after 10 hours of HER in alkaline and acidic electrolytes.
Figure 8 shows the result data of linear sweep voltammetry (LSV) of an electrode according to an embodiment of the present invention.
Figure 9 is an electrochemically active surface area (ECSA), C _dl , and Nyquist plot of an electrode according to an embodiment of the present invention.
Figure 10 shows measured chronopotentiometry, LSV curve, and Faraday efficiency of an electrode according to an embodiment of the present invention.
Figure 11 shows LSV trace and chronopotentiometry data for comparing the performance of a comparative electrode, a commercial Pt/Ni foam electrode, with CoS/S-rGO loaded in NF of an alkaline electrolyte.

Hereinafter, the present invention will be described in more detail.

The present inventors made diligent efforts to develop a stable and durable dual-functional HER electrocatalyst that can operate in both alkaline and acidic electrolytes, and as a result, CoS was added to the CP electrode by hydrothermal synthesis to improve the bond between the CP electrode and catalyst particles. The catalyst was grown through /S-rGO junction, and the CoS/S-rGO/CP electrode prepared in this way not only exhibits high Faradaic efficiency in alkaline and acidic electrolytes with low overpotential and low Tafel slope, but also exhibits high Faradaic efficiency at 10 hours and 3,000 linear degrees. By confirming that excellent HER performance was maintained during the linear sweep voltammetry (LSV) cycle, the present invention was completed by developing an optimal dual-functional HER electrode catalyst with efficient and long stability.

Accordingly, the present invention is reduced graphene oxide (rGO) doped with sulfur element (S); and cobalt sulfide (CoS) particles attached to the rGO. It provides a dual-functional catalyst for hydrogen generation comprising a.

The average XRD crystal lattice distance of rGO doped with the sulfur element (S) may be 3.2 Å to 3.5 Å.

The shape of the particles can be determined by the manufacturing method and chemical properties of the constituents. Accordingly, the shape of the particle is not limited, but may specifically be spherical.

Additionally, the particles may be amorphous, semi-crystalline and/or crystalline. In one embodiment, the particle may include a crystalline portion and an amorphous portion. Specifically, the CoS particles may be crystalline.

In one embodiment of the present invention, the average diameter of the CoS particles may be 0.1 ㎛ to 1 ㎛, specifically 0.1 ㎛ to 0.2 ㎛. The smaller the particle size, the larger the surface area will be, so a larger surface area can increase water access to the catalyst particles during electrolysis.

In the CoS particles, the Co ^{3+ :} Co ²⁺ molar ratio may be 1:(1.8-2.2), specifically 1:2.

The catalyst may include 98 to 99% by weight of CoS particles and 1 to 2% by weight of rGO doped with elemental sulfur (S) based on the total weight of the catalyst.

In addition, the present invention relates to a base material; and a dual-function electrode for hydrogen generation including the catalyst.

The base substrate serves as an electrode for HER, and electrode materials known in the art may be used without limitation, and materials such as known conductors, semiconductors, or insulators may be used.

In one embodiment of the present invention, the base substrate may be a carbon-based support, and specifically, one selected from the group consisting of carbon paper (CP), carbon felt, carbon black, activated carbon, carbon nanofiber, nickel foam, and copper foam. There may be more than one species, and when the carbon-based support is used, agglomeration of the catalysts can be prevented and conductivity can also be increased.

The electrode may have activation of hydrogen evolution reaction (HER) in alkaline electrolyte and acidic electrolyte.

The alkaline electrolyte may be at least one selected from the group consisting of KOH, NaOH, and CsOH, and the acidic electrolyte may be at least one selected from the group consisting of H ₂ SO ₄ , HCl, HNO ₃ and HClO ₄ .

The Tafel slope of the electrode is 80 mV/decade to 100 mV/decade when measured in a wave number range of 2000 to 300 cm ^-1 at an excitation wavelength of 532 nm in 1.0 M aqueous solution of KOH or 0.5 M _aqueous solution of _H 2 SO 4 It could be.

The charge transfer resistance (R _ct ) value of the electrode may be 10 Ω to 20 Ω.

Meanwhile, Figure 1 shows the mechanism for HER using the electrode in alkaline and acidic electrolytes. The mechanism may be expressed as shown in Schemes 1 to 3 below.

[Scheme 1] Volmer step

H ₂ O + e ^- → H _ads (H ⁺ ion absorbed by catalyst) + OH ^-

[Scheme 2] Heyrovsky step

H ₂ O + e ^- + H _ads → H ₂ + OH ^-

[Scheme 3] Tafel step

H _ads + H _ads → H ₂

Referring to Schemes 1 to 3 above, the mechanism is a key half reaction that generates hydrogen at the cathode during water electrolysis and may include a two-electron transfer process. Scheme 1 above is a Volmer step to produce adsorbed hydrogen, which may be followed by a Heyrovsky step (Scheme 2) or a Tafel step (Scheme 3). The Heyrovsky step and Tafel step can proceed simultaneously to produce H ₂ .

The Heyrovsky step reaction is the rate-determining step, and H ⁺ can be adsorbed to the CS sulfur site of S-rGO at the CoS/S-rGO/CP electrode in acidic electrolyte. The H ⁺ can move to CoS through internal diffusion and react by receiving electrons from Co to generate H ₂ gas. At this time, the highly conductive CP can easily receive electrons from an external circuit and increase the electron density around CoS, further promoting this reaction.

Meanwhile, the HER reaction can occur in three stages as shown in Schemes 1 to 3 even in alkaline electrolytes. H _ads , OH _ads (hydroxy adsorption) and dissociation of water are important in alkaline electrolytes for HER activity, and the catalyst may need to be good at adsorbing H ⁺ and also dissociating water well in alkaline electrolytes. The adsorption of water may occur preferentially on CoS in the CoS/S-rGO/CP electrode due to the absence of H ⁺ ions in the alkaline electrolyte. At this time, the adsorbed water can be easily decomposed into OH ^- and H ⁺ due to the strong oxygen evolution reaction (OER) performance of Co. CoS can be reduced to produce H ₂ . However, unlike acidic electrolytes, in CoS in alkaline electrolytes, water decomposition and H ⁺ reduction reactions occur competitively, which can slow down the overall reaction rate, and H ⁺ ions are reduced and adsorbed on the electron-rich S-rGO surface, simultaneously producing H ₂ . can be created.

In addition, when Co ²⁺ is partially oxidized to Co(Ⅲ)SOH through strong interaction with OH ^- ions formed by decomposition of water molecules adsorbed on CoS, HER performance may be reduced in alkaline electrolytes. HER activity is related to H ⁺ adsorption in both alkaline and acidic electrolytes, and the free energy of adsorption of H ⁺ ions (ΔG _H ) is an important factor in hydrogen generation, and appropriate hydrogen bond energy can aid the HER process.

The present invention includes the first step of dispersing graphene oxide (GO) in ethylene glycol (EG) and preparing a GO dispersion; A second step of preparing S-rGO powder by adding a reducing agent to the GO dispersion and stirring it; A third step of mixing the S-rGO powder with distilled water and stirring it to prepare an S-rGO dispersion solution; A fourth step of preparing a first mixed solution by adding a cobalt precursor and a sulfur precursor to the S-rGO dispersion solution and stirring; A fifth step of preparing a second mixed solution by immersing carbon paper (CP) in the first mixed solution and ultrasonicating it; And a sixth step of sealing the CP and the second mixed solution and collecting an electrode containing CoS particles (CoS/S-rGO) attached to rGO doped with elemental sulfur (S) overgrown through hydrothermal synthesis. Provides a method for manufacturing an electrode including;

The concentration of the GO dispersion may be 1 mg/mL to 5 mg/mL, specifically 1 mg/mL to 2 mg/mL.

The reducing agent includes sodium sulfide (Na ₂ S), ammonium thiocyanate ([NH ₄ S]CN), thiourea ((NH ₂ ) ₂ CS), and thiourea dioxide; (NH)(NH ₂ )CSO ₂ H) and ethane-1,2-dithiol (1,2-Ethanedithiol; C ₂ H ₄ (SH) ₂ ) It may be any one selected from the group consisting of, specifically It may be sodium sulfide (Na ₂ S).

The cobalt precursor is selected from the group consisting of cobalt chloride (CoCl ₂ ·6H ₂ O), cobalt nitrate (Co(NO ₃ ) ₂ ·6H ₂ O), and cobalt acetate ((CH ₃ COO) ₂ Co·4H ₂ O) There may be one or more selected types, and specifically, it may be cobalt chloride (CoCl ₂ ·6H ₂ O).

The concentration of the cobalt precursor may be 0.01mM to 1mM, specifically 0.01mM to 0.05mM.

The sulfur precursor includes sodium sulfide (Na ₂ S), thiourea (CH ₄ N ₂ S), ammonium thiocyanate (NH ₄ SCN), thioacetamide (C ₂ H ₅ NS), Benzyl disulfide (C ₁₄ H ₁₄ S ₂ ), thiophene (C ₄ H ₄ S), 2,2'-dithiophene (2,2'-dithiophene; C ₈ H ₆ S ₂ ), 1 selected from the group consisting of p-toluenesulfonic acid (C ₇ H ₈ O ₃ S), 2-thiophenemethanol (C ₅ H ₆ OS), and sulfur powder (S powder) It may be more than one species.

In the sixth step, hydrothermal synthesis may be performed at a temperature of 150 to 200 °C for 8 to 10 hours.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

Hereinafter, the present invention will be described in more detail through specific examples.

<Example> Preparation of HER catalyst and electrode

Example 1. Preparation of Co/CP electrode

First, Co crystals were grown on the CP base electrode. CoCl ₂ ·6H ₂ O (3.2 mmol, 99.9 %, Sigma-Aldrich) was mixed with 60 mL of ethylene glycol (EG, C ₂ H ₆ O ₂ , 99.9 %, Sigma-Aldrich) and stirred for 1 hour. 1.0 g of poly(vinylpolypyrrolidone) (PVP(C ₆ H ₉ NO) _n , Sigma-Aldrich) as a dispersant and 1.0 mL of reducing agent were added and stirred for 1 hour. At this time, the reducing agent may be one or more selected from the group consisting of hydrazin hydrate (N ₂ H ₄ , 99.9%, Sigma-Aldrich), NaBH ₄ , ascorbic acid, and LiAlH ₄ . Afterwards, _carbon paper (CP, 1.0 After removal, it was completely sealed. The temperature of the autoclave was raised to 190°C at a rate of 10°C/min and then maintained at that temperature for 10 hours. After cooling to room temperature, the container was opened, the Co-overgrown CP electrode was recovered, washed three times with distilled water and ethanol, and dried in a drying oven at 70°C for 5 hours to obtain a Co/CP electrode.

Example 2. Preparation of rGO/CP electrode

0.6 g of graphite oxide (GO) prepared by modified Hummers' method was dispersed in 60 mL of EG and stirred for 3 hours. Then, 1.0 mL of reducing agent was added, and the mixture was sonicated for 1 hour, and then the solution was transferred to an autoclave vessel and purged three times with N ₂ gas to remove internal gas and completely sealed. At this time, the reducing agent may be one or more selected from the group consisting of hydrazine hydrate, NaBH ₄ , ascorbic acid, and LiAlH ₄ . Afterwards, the temperature of the autoclave was increased from 10°C per minute to 180°C and maintained at that temperature for 16 hours. The autoclave was cooled to room temperature, the container was opened, the rGO-overgrown CP electrode was recovered, washed at least three times with distilled water and ethanol, and then dried in an oven at 70 °C for 5 hours to obtain an rGO/CP electrode.

Example 3. Preparation of S-rGO/CP electrode

The S-rGO/CP electrode was prepared in the same manner as the rGO/CP electrode, except that 0.15 g of 2.0 mmol Na ₂ S was used as a reducing agent.

Example 4. Preparation of CoS/CP electrode

3.2 mmol of CoCl ₂ ·6H ₂ O was added to 60 mL of distilled water, and the mixture was stirred for 30 minutes. Next, 8.0 mmol of thiourea (CH ₄ N ₂ S, 99.99 %, Sigma-Aldrich) was added, and the mixed solution was stirred for 1 hour. CP was immersed in the mixed solution and sonicated for 1 hour. Next, the mixed solution was transferred to an autoclave and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased to 190 °C at a rate of 10 °C/min and then maintained at that temperature for 24 hours. After the autoclave was cooled to room temperature, the container was opened to recover the CoS overgrown CP electrode, washed at least three times with distilled water and ethanol, and then dried in a drying oven at 70°C for 5 hours to obtain a CoS/CP electrode. .

Example 5. Preparation of Co/S-rGO/CP electrode

0.6 g of graphite oxide (GO) prepared by the modified Hummers' method was dispersed in 60 mL of EG and stirred for 3 hours. Then, 0.15 g of 2.0 mmol of Na ₂ S was added, and the mixture was sonicated for 1 hour, and then the solution was transferred to an autoclave vessel and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased from 10 °C per minute to 180 °C and maintained at that temperature for 16 hours. The autoclave was cooled to room temperature, the container was opened to recover the S-rGO powder, washed at least three times with distilled water and ethanol, and dried in an oven at 70°C for 5 hours to obtain S-rGO powder. After adding S-rGO powder to 60 mL of EG, the mixture was sonicated for 1 hour. Next, 3.2 mmol of CoCl ₂ ·6H ₂ O was added and stirred for 1 hour, then 1.0 g of poly(vinylpolypyrrolidone) as a dispersant and 1.0 mL of reducing agent were added and stirred for 1 hour. CP was immersed in the mixed solution and sonicated for 1 hour. Next, the mixed solution was transferred to an autoclave and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased to 190 °C at a rate of 10 °C/min and then maintained at that temperature for 24 hours. After cooling the autoclave to room temperature, open the container to recover the Co/S-rGO overgrown CP electrode, wash it at least three times with distilled water and ethanol, and dry it in a drying oven at 70°C for 5 hours to obtain Co/S-rGO. -rGO/CP electrode was obtained.

Example 6. Preparation of CoS/rGO/CP electrode

0.6 g of graphite oxide (GO) prepared by modified Hummers' method was dispersed in 60 mL of EG and stirred for 3 hours. Subsequently, the GO dispersion was sonicated for 1 hour, and then the solution was transferred to an autoclave container and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased from 10 °C per minute to 180 °C and maintained at that temperature for 16 hours. The autoclave was cooled to room temperature, the container was opened, the rGO-overgrown CP electrode was recovered, washed at least three times with distilled water and ethanol, and then dried in an oven at 70 °C for 5 hours to obtain an rGO/CP electrode. Then, 3.2 mmol of CoCl ₂ ·6H ₂ O was added, and after stirring for 30 minutes, 8.0 mmol of thiourea (CH ₄ N ₂ S, 99.99 %, Sigma-Aldrich) was added, and the mixed solution was stirred for 1 hour. The rGO/CP electrode was immersed in the mixed solution and sonicated for 1 hour. Next, the mixed solution was transferred to an autoclave and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased to 190 °C at a rate of 10 °C/min and then maintained at that temperature for 24 hours. After cooling the autoclave to room temperature, open the container to recover the CoS/rGO overgrown CP electrode, wash it at least three times with distilled water and ethanol, and dry it in a drying oven at 70°C for 5 hours to obtain CoS/rGO/CP. An electrode was obtained.

Example 7. Preparation of CoS/S-rGO/CP electrode

0.6 g of graphite oxide (GO) prepared by the modified Hummers' method was dispersed in 60 mL of EG and stirred for 3 hours. Then, 0.15 g of 2.0 mmol of Na ₂ S was added, and the mixture was sonicated for 1 hour, and then the solution was transferred to an autoclave vessel and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased from 10 °C per minute to 180 °C and maintained at that temperature for 16 hours. The autoclave was cooled to room temperature, the container was opened to recover the S-rGO powder, washed at least three times with distilled water and ethanol, and dried in an oven at 70°C for 5 hours to obtain S-rGO powder. After adding S-rGO powder to 60 mL of distilled water, the mixture was sonicated for 1 hour. Then, 3.2 mmol of CoCl ₂ ·6H ₂ O was added, and after stirring for 30 minutes, 8.0 mmol of thiourea (CH ₄ N ₂ S, 99.99 %, Sigma-Aldrich) was added, and the mixed solution was stirred for 1 hour. CP was immersed in the mixed solution and sonicated for 1 hour. Next, the mixed solution was transferred to an autoclave and purged three times with N ₂ gas to remove internal gas and completely sealed. The temperature of the autoclave was increased to 190 °C at a rate of 10 °C/min and then maintained at that temperature for 24 hours. After cooling the autoclave to room temperature, open the container to recover the CoS/S-rGO overgrown CP electrode, wash it at least three times with distilled water and ethanol, and dry it in a drying oven at 70°C for 5 hours to obtain CoS/S-rGO. -rGO/CP electrode was obtained.

Comparative Example 1

A Pt/Ni foam electrode, a commercial catalyst, was prepared.

<Experimental Example 1> Evaluation of physicochemical properties of catalysts according to Examples 1 to 7

X-ray diffraction (XRD) patterns of the catalysts according to Examples 1 to 7 grown directly on the CP electrode were obtained by incident Ni-filtered CuKα radiation (30.0 kV, 15.0 mA, λ = 1.5419 Å) at 10 to 90° 2θ. recorded in the range. The morphology of these catalysts was examined using a scanning electron microscope (SEM, S-4100, HITACHI, Japan), and energy dispersive spectroscopy (EDS) was used for elemental analysis. The surface elemental composition of the catalyst was measured by X-ray photoelectron spectroscopy (XPS, AXIS-NOVA, Kratos Inc., USA) using A1 Kα (1486.8 eV) radiation. The lattice structure of the CoS/S-rGO catalyst grown directly on the CP electrode was determined using a high-resolution transmission electron microscope (HR-TEM, JEOL-7001F), and the rGO, SrGO, and CoS/S-rGO catalysts grown directly on the CP electrode were determined. It was analyzed by Raman spectroscopy (NRS-3300, JASCO) in the wavenumber range of 2000 to 300 cm ^-1 at an excitation wavelength of 532 nm. When evaluating the Faradaic efficiency of the catalyst, the reactor was purged with Ar and the generated gas was analyzed every 30 minutes by gas chromatography (DS-7200, DS Science, Korea) using a thermal conductivity detector.

1-1. Observation of XRD patterns of electrodes according to Examples 1 to 7

Figure 2 shows the XRD pattern, Raman spectrum, and FT-IR spectrum of electrodes according to Examples 1 to 7 prepared by hydrothermal synthesis. In particular, Figures 2a-1 and 2a-2 are XRD patterns of electrodes according to Examples 1 to 7 manufactured by hydrothermal synthesis. All electrodes showed peaks at 2θ = 26.34°, 42.5°, and 54.23°, corresponding to the (002), (100), and (004) diffraction planes of a typical hexagonal graphite CP (JDPDS#08-8812). According to the Bragg equation, the lattice distance (d-value) increases with increasing O, so the (002) diffraction peak shifts to a lower angle and becomes slightly less intense when O in S-rGO is partially replaced by S. was confirmed. Lower angle peaks were also observed in the XRD patterns of Example 5 and Example 6 electrodes. While lattice distance changes were observed due to crystal distortion due to crystal defects, the d-value of the electrode of Example 7 was recovered, which may mean that CoS bound to S-rGO compensated for the defects of rGO. In other words, during synthesis, CoS may fill additional vacancies in S-rGO, resulting in fewer defects.

Meanwhile, the peaks observed at 2θ=44.37°, 51.28°, 75.37° (JCPDS#01-089-4307) corresponding to the (111), (200), and (220) diffraction planes are the cubic Co metal in Example 1 and Example 5 Observed at the electrode, peaks at 2θ = 30.47°, 35.26°, 46.66° and 54.09° are (100), (101), (102), (110) Example 4, Example 6 and Example 7 A diffraction surface of hexagonal CoS (space group: P63/mmc) at the electrode was observed in the XRD pattern (JCPDS# 03-065-3418). In particular, the main (002) diffraction peak observed for rGO and S-rGO was much less intense after Co or CoS was loaded. Additionally, it can be seen that the intensities of the (111) and (100) diffraction peaks corresponding to Co and CoS are significantly lower than other peaks after loading. Based on these results, it can be predicted that the (111) plane of Co and the (100) plane of CoS will be loaded on the (001) plane of rGO or S-rGO.

1-2. Observation of Raman spectra of electrodes according to Examples 1 to 7

Figures 2b-1 and 2b-2 are Raman spectra of carbon in Example 2, Example 3, and Example 7 electrodes and Co and CoS in Example 1 and Example 4 electrodes, respectively. The two split peaks in the hexagonal 2D graphitic carbon spectrum may be due to the interaction of π-electrons dissipating energy between adjacent graphitic units. That is, the two peaks observed around 1330 and 1600 cm ^-1 may correspond to the D and G dense bands, respectively. The D band represents the mode of motion of k-point photons with A _1g symmetry, while the G band may be related to the first-order scattering of E _2g phonons by sp ² carbon atoms. Typically, the D band represents disorder (defects) in amorphous and quasicrystalline forms of carbon materials, while the G band may represent graphite crystallinity. The IG/ID intensity ratios for carbon of rGO, S-rGO, and CoS/S-rGO were determined to be 1.06, 1.03, and 0.94, respectively, which are close to 1 and may indicate highly crystalline graphite. A lower ratio may indicate higher carbon crystallinity. In particular, the electrode of Example 7 with an IG/ID intensity ratio of 0.94 can exhibit the highest carbon crystallinity. The intensity of the crystalline G band generally increases with increasing carbon layers, while physical or chemical processes that form defects in pure graphite can be identified by the appearance of the D band. As shown in the caption, the cleaved C=C bond is connected to a C=S bond, and S, which has a large radius, is replaced by the O site of rGO, reforming the hexagonal structure, allowing the bond to partially disappear. Additionally, the number of defects can be lowered by CoS loading in the remaining defect sites of S-rGO.

Meanwhile, the Co spectrum of the electrode of Example 1 may be A _g , E _g , F _12g , F _22g and A _1g modes of typical cubic Co metal. Additionally, peaks corresponding to the E _g , F _2g and A _1g symmetry modes of hexagonal CoS were observed at 475, 517 and 676 cm ⁻¹ . The peak is known to shift to higher wave numbers when grafted to other compounds. The CoS peak may shift to a higher wavenumber when loaded into CP. FTIR spectroscopy can be a fast and non-destructive tool to determine the shared functionality of S-rGO. The spectrum shown in Figure 2c can show that rGO exhibits C=C and CH stretching bands at 1639 and 1863∼2926 cm ^{-1 ,} corresponding to the rings of hexagonal graphite. The bands observed at 1086 and 1710 cm ^-1 may correspond to the CO and C=O vibration modes of the defect site, respectively. In contrast, S-substituted S-rGO exhibited bands at 874 and 801 cm ^-1 , which correspond to the stretching vibration of the CS moiety inserted into the hexagonal graphite ring and the OS bending vibration associated with S bound to unreduced O, respectively. It may correspond to . The peak is also clearly visible in the spectrum of CoS/S-rGO, although somewhat less intense. This may be due to S bonding to the defective C in the hexagonal graphite ring of S-rGO, forming a complete hexagonal array connected by C=S, as shown in the caption of Figure 2b-1.

1-3. SEM image observation of electrodes according to Examples 1 to 7

Figure 3 is an SEM image and EDS spectrum of an electrode according to an embodiment of the present invention. The SEM image in Figure 3a shows the size and shape of rGO, S-rGO, Co, CoS, Co/S-rGO, CoS/rGO, and CoS/S-rGO particles grown on CP base electrode. Example 2 The electrode was observed to be composed of rGO sheets with a size of 1.0 × 0.2 μm uniformly grown on CP. On the other hand, the Example 3 electrode consists of a larger sheet of 2.0 You can. Example 4 In the electrode, particles with a diameter of 5 to 7 ㎛ may have grown in CP. Moreover, in the Example 5 electrode, very small spherical particles appear to have grown uniformly in the CP, whereas the Example 6 electrode showed rearranged CoS dodecahedrons with panels measuring 1.0 × 4.0 μm, with the dodecahedrons centered at the center. They may be combined to form a cross shape that completely covers the CP. Finally, the electrode of Example 7 may be composed of rearranged particles that grow in a flower shape covering the CP. However, unlike CoS/rGO, in this case even the particle junctions can be completely filled and appear more dense.

1-4. Observation of EDS spectra of electrodes according to Examples 1 to 7

Figure 3b shows EDS spectra of elements present on the electrode surfaces of Examples 1 to 4, and their atomic compositions are shown in Table 1 below.

[Table 1]

For the Example 2 electrode, a peak corresponding to C present in rGO and CP was observed, and a peak corresponding to O present in the remaining rGO was also observed. The C:O ratio was measured as 85.72:14.29. The Example 3 electrode showed peaks corresponding to C, a small amount of S, and unsubstituted O present in S-rGO at a C:S:O ratio of 87.35:5.11:7.54, while the spectrum of the Example 1 electrode showed It was determined that C, Co, and Co were loaded into CP at a rate of approximately 30 at%. Finally, peaks corresponding to Co, S, and C were observed in the electrode spectrum of Example 4, and the Co:S ratio was quantitatively confirmed to be about 1:1 (i.e., 18.74:17.84). In addition, no other impurities were observed, and it can be seen that the sample used in the experiment was stably synthesized.

1-5. SEM image observation of the electrode of Example 7 before and after HER

Figure 4 is an SEM image of the Example 7 electrode surface after 3,000 CV cycles in 1.0 M KOH and 0.5 MH ₂ SO ₄ . Referring to Figure 4, it can be seen that the CoS/S-rGO catalyst is not separated from the CP surface even after 3,000 cycles in both alkaline and acidic electrolytes and remains stably attached to the electrode. In particular, the catalyst appeared to attach more stably on the electrode used in the acidic electrolyte, and clusters of newly grown grains were observed in the CP, which was attributed to the high OER activity of Co, which is associated with the strong OH ^- functionalization of the alkaline electrolyte. The partial oxidation of Co2 ⁺ of CoS may result in the formation of Co(Ⅲ)SOH species, but because S-rGO acts as a blocking layer, such particles are not generated in large numbers, and as a result, the HER performance is quite good. maintained. Additionally, the amounts of Co and S present on the electrode surface were measured to determine whether Co and S were eluted from the electrode surface over 3,000 CV cycles. The S:Co atomic ratio of the electrode used in the alkaline electrolyte was found to be 47.81:52.19, which may be different from the 47.33:52.67 ratio of the initial electrode. 0.52 of Co was lost during the cycle. It can be seen that the Co:S atomic ratio of 1:1.09 was maintained at a stoichiometric ratio of 1:1. Meanwhile, the S:Co atomic ratio was determined to be 46.69:53.31 after 3,000 cycles in the acidic electrolyte solution, indicating that the Co level decreased by about 0.44 from the initial value and could be almost negligible. Through these results, it can be confirmed that CoS does not dissolve in the electrode of Example 7 during the reaction.

1-6. High-resolution Transmission Electron Microscopy (HRTEM) observation of the electrode according to Example 7

CoS/S-rGO particles were carefully scraped from the Example 7 electrode and examined by HRTEM, as shown in Figure 5. Figure 5 is an HRTEM image of the electrode in Example 7 of the present invention. Referring to Figure 5a, it can be seen that the CoS crystal is spherical in shape and hexagonal in size about 0.1 to 0.2 ㎛. In addition, the CoS crystals appear to be gathered in groups of 3 or 4 on a thin S-rGO sheet, indicating that the CoS crystals are well loaded on each S-rGO sheet. Additionally, it was observed that the lattice distance of the (002) plane of the S-rGO sheet increased significantly due to S substitution.

Figure 5b shows the lattice distances of 2.92, 1.67, 1.94, and 2.56 Å corresponding to the (100), (110), (102), and (101) crystal planes of CoS, respectively. Meanwhile, Figure 5c is the selected area electron diffraction (SAED) pattern of CoS/S-rGO, which shows the presence of (100), (110), (102), and (101) CoS planes identified by XRD. was confirmed. When the points on this diffraction surface were connected in FFT, a hexagonal shape was obtained, which was predicted to be a single crystal. Meanwhile, the S-rGO sheet showed a SAED pattern consisting of scattered and ring-shaped patterns, indicating homogeneous polycrystalline. Additionally, S-rGO has an XRD crystal lattice distance of 3.38 Å, which may be slightly larger than rGO (3.11 Å).

The high-angle annular dark field (HAADF) elemental map in Figure 5d can show that the Co and S elements are evenly distributed in the CoS particles, and the carbon sheets are clearly observed spread below. However, it is difficult to confirm whether ion-exchanged S is present, so the results of applying EDS only to the S-rGO part are shown in Figure 5e. From this, the C:S atomic ratio was determined to be 95.65:4.34, and S is responsible for O in rGO. You can see that it has been partially replaced.

1-7. XPS observation of electrodes according to Examples 1 to 7

Using XPS, we qualitatively investigated how the oxidation state of the catalyst component changes depending on the configuration of the electrode, and this is shown in Figure 6. Figure 6 shows the result data of X-ray photoelectron spectroscopy (XPS) of an electrode according to an embodiment of the present invention. Referring to Figure 6, the shoulder peak at 285.5 eV was strengthened and broadened when CP was loaded with rGO, S-rGO, Co, or CoS, which may be consistent with the change in C chemical environment. Meanwhile, the 284.8/285.5 eV split peak observed in the CP spectrum was observed to merge into one broad peak at a lower binding energy in the electrode spectra of Examples 5 to 7. This may mean that loading Co or CoS into rGO and S-rGO further reduces the carbon of CP. Meanwhile, the peak at 286.0 eV may be due to the carbon present in rGO, while the peak at 288.0 eV may correspond to the carbon present in S-rGO. Additionally, when CoS was added, a very small peak that was difficult to identify was observed at 288.0 eV. Compared to the C(1s) peak in the spectrum of the Example 6 electrode, the corresponding peak due to rGO in the spectrum of the Example 7 electrode was observed at a slightly higher binding energy, which means that the C of rGO is higher in S than in CoS alone. -Can be loaded into rGO. In other words, the C=C electrons of S-rGO may be moving to CoS. On the other hand, the electrodes of Example 3 and Example 5 only show the S(1s) peak at 169.1 eV due to the presence of C=S, while the electrodes of Example 4, Example 6, and Example 7 show only the S(1s) peak at 161.7 eV and 162.7 eV. Peak splitting may be seen. The peaks converge to the 2p _3/2 and 2p _1/2 peaks of S, respectively, and may be due to the S ^2- species of CoS. S-rGO can be further reduced by loaded Co metal and CoS. Moreover, S in CoS can be further oxidized upon loading rGO and S-rGO, as evidenced by the higher binding energy. Therefore, the S electrons of CoS can move to C=S of S-rGO through rGO and S-rGO loading. Meanwhile, in the spectrum of the electrode of Example 1, Co 2p _1/2 and 2p _3/2 peaks were observed at about 797.4 and 796.0 eV, 781.1 and 779.9 eV. These converge to Co ⁰ and Co ²⁺ peaks, respectively, and most of them may exist as Co ⁰ . This peak was observed at a higher binding energy (oxidation) in the electrode spectrum of Example 5, and was richer in Co ²⁺ than the electrode of Example 1, so the electrons of Co metal may have been transferred to S-rGO through loading. The Example 4 electrode exhibited separate 2p _1/2 and 2p _3/2 peaks at 796.3 and 793.6 eV, 780.0, and 778.6 eV, respectively, due to the presence of Co ²⁺ and Co ³⁺ . These peaks were observed at lower binding energies in the electrode spectra of Examples 6 and 7, which may mean that electrons in rGO and S-rGO move to Co in CoS upon loading. S in S-rGO donates electrons to Co ²⁺ in CoS through the CoS/S-rGO junction of the Example 7 electrode, while S in CoS can push electrons to C=S in S-rGO. This may be because the electrons on the surface of the CoS/S-rGO/CP bonded particles move in the following order: (S-rGO)C=C, CoS, S=C(S-rGO). Also, at this time, the conductivity can be further increased by reducing CP, which is the base electrode. Therefore, in the electrode of Example 7, which has a higher electron density due to donation from S of CoS in an acidic solution, H ⁺ ions are adsorbed on the C=S sulfur site of S-rGO, receive electrons from highly conductive CoS, and H ₂ production can be easily accomplished. On the other hand, since there is no H ⁺ in alkaline electrolyte, CoS can preferentially adsorb water. The Co-S bond strength can decrease as the electron density of S HOMO decreases and the electron density of LUMO increases in CoS. Therefore, moisture adsorption becomes easier, and the adsorbed moisture receives electrons from the highly conductive CP and can be easily decomposed to generate H ⁺ . These H ⁺ ions can be easily reduced by adsorption to the C=S sulfur atom of S-rGO, which has a high electron density, as in an acidic solution, and generate hydrogen. On the other hand, due to Co/S-rGO bonding, Co electrons move from the electrode of Example 5 to S-rGO, increasing the electron density of the C=S bond in S-rGO, which may cause moisture adsorption through hydrogen bonding. However, because Co loses some of its metallic properties and the conductivity of CP hardly increases, H ⁺ is more difficult to reduce than in the Example 7 electrode, which may result in low hydrogen productivity.

1-8. Observation of XRD, Raman spectroscopy and XPS data of CoS/S-rGO electrocatalyst

Figure 7 shows the XRD pattern, Raman spectrum, and XPS data for CoS/S-rGO scraped from CP before and after 10 hours of HER in alkaline and acidic electrolytes. In the XRD pattern of Figure 7a, the peak positions before and after the reaction were the same, so there was no crystal collapse, but the overall intensity decreased after the reaction. In particular, because the intensity of the peak observed after use in an alkaline electrolyte is lower than after use in an acidic electrolyte, the CoS of the electrode of Example 7 may be more stable in an acidic electrolyte than in an alkaline electrolyte.

Meanwhile, Figure 7b showed that A _1g , F _2g , and E _g Raman peaks corresponding to the characteristic symmetry mode of CoS and D and G bands corresponding to the carbon of S-rGO were still observed. In addition, after the reaction, a sharp CoS peak is observed 10 hours after HER in the acidic electrolyte, while the D band corresponding to the carbon of S-rGO broadens and the G band becomes sharp compared to before the reaction. This may indicate that CoS maintained its crystallinity stably even during the 10-hour reaction, while the carbon defects present in S-rGO were simultaneously reduced. On the other hand, after 10 hours of reaction in an alkaline electrolyte, the intensity of the characteristic CoS peak becomes slightly weaker, the carbon D band almost disappears, and the G band becomes sharper, which may make CoS less stable in the alkaline electrolyte. While the crystallinity of carbon in S-rGO was improved, referring to Figure ^9c , which is ^the 2p _1/2 and 2p _3/2 peaks can be shown. The ratio of Co ²⁺ /Co ³⁺ changed during the reaction and decreased further after the reaction in the alkaline electrolyte compared to the acidic electrolyte. In other words, as Co(III)SOH is generated in the electrode reacted in the alkaline electrolyte, Co ²⁺ may be oxidized and the abundance ratio of Co ³⁺ may increase. As a result, the electrode reacted in the acidic electrolyte contains more Co ²⁺ after the reaction than before the reaction, which may be due to the occurrence of H ₂ evolution that reduces CoS. Meanwhile, C _1s peaks corresponding to various carbon species were observed in the S-rGO spectrum before the reaction converging to CC, CS, CO, and C=O species. While the positions of these peaks were almost the same after the reaction, the intensity of the CS peak, which indicates the crystallinity of carbon in S-rGO, was found to be stronger after the reaction in alkaline electrolyte. This indicates that S in CoS was reduced during reaction in the acidic electrolyte. That is, electrons transferred from the external circuit are collected in CoS of the acidic electrolyte, and H ⁺ is eventually reduced in CoS and converted to H ₂ . On the other hand, the same S 2p _3/2 peak was observed at higher binding energy after reaction in alkaline electrolyte, which may be due to OH ⁻ generated from the number of molecules decomposed from the remaining CoS adsorbed on CoS. As a result, Co ²⁺ may be partially oxidized to Co ³⁺ .

<Experimental Example 2> Evaluation of electrochemical properties of catalysts according to Examples 1 to 7

Electrochemical experiments were performed using an electrochemical cell test system (IVIUMnSTAT, Ivium Technologies, Netherlands) at room temperature. The electrochemical HER activity of each catalyst was measured in alkaline (1.0M KOH) and acidic (0.5M, H ₂ SO ₄ ) media using a standard three-electrode electrochemical apparatus consisting of the electrodes of Examples 1 to 4. CP, the electrodes of Examples 5 to 7 were used as a working electrode, a saturated calomel electrode (Hg/Hg ₂ Cl ₂ , SCE) was used as a reference electrode, and a Pt wire was used as a counter electrode. 1.0 M KOH solution (or 0.5 MH ₂ SO ₄ ) was used as electrolyte. The reversible hydrogen electrode potential (VRHE) was determined from the measured potential using Equation 1 below.

[Equation 1]

VRHE = VSCE + 0.241 + 0.059 × pH

Electrode polarization curves were recorded by LSV at 5.0 mV/s in 1.0 M KOH (or 0.5 MH ₂ SO ₄ ) solution with Tafel plots derived from the obtained polarization curves. The Tafel slope (b) for each electrode was calculated from the linear region of each plot using Equation 2 below.

[Equation 2]

η=a + blogj

(where η is the potential difference, j is the current density, a is the electrode material dependent constant, and b is the Tafel slope)

The electrodes of Examples 1 to 7 were analyzed by electrochemical impedance spectroscopy (EIS) in the frequency range from 100 kHz to 0.01 Hz at an applied AC voltage of 5 mV. The long-term stability and durability of each electrode were evaluated by chronopotentiometry at a constant current density of 100 mA/cm. The electrochemical double layer capacitance (C _dl ) of the electrodes of Examples 1 to 7 was measured by voltammetry (CV) at 20, 40, 60, 80, and 100 mV/s in the non-Faraday region, and the obtained electrochemical data were calculated as the current Interruptions were automatically corrected using iR compensation.

2-1. LSV measurement of electrodes according to Examples 1 to 7

The electrochemical performance of the electrodes according to Examples 1 to 7 was evaluated in 1.0 M KOH and 0.5 MH ₂ SO ₄ electrolyte. Figure 8 shows result data of LSV according to an embodiment of the present invention. Figure 8a shows the potential scanning form of the LSV of the electrode in the potential range of 0.2 V to -0.6 V. The current for the rGO/CP electrode did not reach 100 mA/cm ² for both electrolytes within the measured voltage range, which may indicate that the Example 2 electrode functions only marginally as a catalyst. Meanwhile, the electrodes of Examples 1 and 3 to 7 had a current density of 100 mA/cm ² and -0.57 V (η = 685 mV), -0.35 V (η = 569 mV), -0.32 in 1.0 M KOH electrolyte, respectively. Overpotentials of V (η = 310mV), -0.27V (η = 248mV), -0.25V (η = 274mV), and -0.22V (η = 218mV) were shown, which indicates that the electrode of Example 7 is suitable for HER in KOH. It can indicate that it can perform best. In contrast, the electrodes achieved -0.54 V (η = 622 mV), -0.32 V (η = 540 mV), -0.24 V (η = 293 mV), _respectively , at a current density of 100 mA/cm ² and 0.5 MH ₂ SO 4 electrolyte. It showed overpotentials of -0.29V (η = 323mV), 0.23V (η = 230mV), -0.24V (η = 244mV), and -0.21V (η = 203mV), and the electrode of Example 7 showed HER even in acidic electrolyte. It can be confirmed that it shows the best performance. In addition, it can be confirmed that the overpotential is slightly lower in acidic electrolytes than in alkaline electrolytes, showing better HER performance in acidic electrolytes.

Figure 8b is the Tafel slope determined by LSV at each electrode. Electrodes with lower Tafel slopes are generally known to have lower exchange current densities and lower overpotentials for HER, which may result in improved catalytic performance. The electrodes according to Preparation Examples 1 to 7 exhibit Tafel slopes of 160.1, 158.7, 128.1, 136.3, 103.6, 112.8, and 96.1 mV/dec, respectively, in KOH electrolyte, and the electrode of Example 7 has the smallest Tafel slope, showing excellent HER catalytic activity. It can be expressed. On the other hand, the electrodes according to Preparation Examples 1 to 7 exhibit Tafel slopes of 153.7, 147.4, 118.8, 126.2, 99.7, 101.3, and 89.2 mV/dec in H ₂ SO ₄ electrolyte, and the electrode of Example 7 has a Tafel slope even in acidic electrolyte. It was found to be the smallest. In particular, the Tafel slope of the acidic electrolyte is 6.9 mV/dec smaller than that of the alkaline electrolyte, which may indicate the highest HER catalytic activity.

2-2. Electrochemically active surface area (ECSA) and double layer capacitance (C) of electrodes according to Examples 1 to 7 _dl ) Measurement of

Figure 9 is ECSA, C _dl and Nyquist plots of electrodes according to Preparation Examples 1 to 7. Since C _dl is proportional to ECSA, it can be determined from the ECSA value. The ECSA of an electrocatalyst can be directly related to its activity since the reaction generally occurs on the surface of the catalyst in solution. C _dl was measured in the non-Faraday region, where the catalyst does not undergo redox chemistry.

Referring to Figure 9a, it can be seen that the difference between current density (ΔJ) and ESCA (XPS) obtained at 20 to 100 mV/s shows a linear trend. All electrodes showed wider CV curves in acidic electrolytes than in alkaline electrolytes, and in particular, the Example 7 electrode showed the widest CV curves in both acidic and alkaline electrolytes. The width of the CV curve of each electrode was plotted with ESCA and expressed as C _dl value. The electrode of Example 7 showed the highest C _dl value of 101.4 mF/cm ² . Taken together, these results indicated that the electrode of Example 7 had excellent electrocatalytic activity due to its high double layer capacitance.

2-3. Observation of Nyquist plots of electrodes according to Examples 1 to 7

9B is a Nyquist plot of electrodes according to Examples 1 to 7. The y-axis represents impedance related to the recombination rate between charges inside the electrode, and the higher the recombination rate, the greater the loss of charge inside the electrode. On the other hand, the x-axis represents the impedance associated with charge movement on the electrode surface, and charge movement can generally be impeded by increasing resistance. As a result, electrodes with smaller semicircles have lower recombination rates and lower resistances, which can exhibit superior electrical conductivity. Here, R _s , R _ct , and R _dt may represent the solution resistance, charge transfer resistance, and charge of the double-layer capacitor, respectively. The most important parameter may be R _ct , which is determined by the size of the semicircle in the Nyquist plot. R _s was observed to be approximately the same for each electrode in each electrolyte, whereas R _ct varied. In particular, all electrodes showed significantly lower R _ct values in acidic electrolyte than in alkaline electrolyte. The electrodes according to Examples 1 to 7 showed R _ct values of 34.5, 32.0, 24.0, 23.0, 17.5, 15.5, and 13.5 Ω, respectively, in alkaline electrolyte, and 30.0, 28.0, 22.0, 18.5, 14.0, 12.0, and It showed a R _ct value of 11.0 Ω. In particular, the electrode of Example 7 can exhibit the lowest resistance in both alkaline and acidic electrolytes and thus the highest charge transfer efficiency.

2-4. Evaluation of long-term durability of electrodes according to Examples 1 to 7

Chronopotentiometry is an important method for assessing the long-term durability of electrodes and can provide an indicator of electrode stability over time. Figure 10 shows measured chronopotentiometry, LSV curve, and Faraday efficiency of an electrode according to an embodiment of the present invention. Figure 10a is a graph comparing the time potential difference measurements for Examples 1 to 7 at 100 mA/cm ² for 10 hours. All electrodes required up to 7,500 seconds to stabilize in the 1.0 M KOH electrolyte, after which a constant overpotential was maintained, with the Example 7 electrode being observed to maintain a stable overpotential of -0.24 V for 35,000 seconds. Meanwhile, the electrode was stabilized slightly faster in the 0.5 MH ₂ SO ₄ acidic electrolyte than in the alkaline electrolyte KOH, and a more gradual stabilization slope was observed. In particular, the electrode of Example 7, which performed the best, maintained an overpotential of -0.21 V from the start of the experiment to 35,000 seconds. These results show that the electrode of Example 7 is optimally stable in both alkaline and acidic electrolytes.

Figure 10b shows the results of repeatedly applying LSV to the electrode of Example 7, which had the best catalytic activity. The electrode overpotential gradually shifts to the left and becomes negative in the alkaline electrolyte over the first 3,000 cycles, at which point it reaches -0.24 V, -0.02 V away from the initial value. On the other hand, the overpotential was more negative in the acidic electrolyte than in the alkaline electrolyte during the first 100 cycles, but no further increase was observed at the value of -0.215 V by 3,000 cycles. The negligible difference from the initial overpotential of 0.015 V shows that the Example 7 electrode is very durable in acidic electrolyte.

Figure 10c is the Faradaic efficiency determined for the Example 7 electrode in alkaline and acidic electrolytes based on actual and theoretical hydrogen evolution at 100 mA/cm ² . The theoretical amount of hydrogen generated and the Faraday efficiency of the electrode were calculated using equations such as Equation 3 below.

[Equation 3]

Faradaic efficiency (%)=znF/It×100 %

(where z is the number of exchanged electrons, n is the number of moles of the desired product, I is the current, t is the electrolysis reaction time of water, and F is the Faraday constant of 96,485.3 C/mol)

The Example 7 electrode provided Faradaic efficiencies of 97.01% and 97.85% in alkaline and acidic electrolytes, respectively, with better HER performance for acidic electrolytes. This may mean that 97% or 98% of the electrons used were involved in HER in both alkaline and acidic electrolytes without side reactions.

2-5. Performance evaluation of electrodes according to examples and comparative examples

Figure 11 shows LSV trace and chronopotentiometry data for comparing the performance of a comparative electrode, a commercial Pt/Ni foam electrode, with CoS/S-rGO loaded in NF of an alkaline electrolyte. Referring to Figure 11, the Pt/NF electrode loaded with 20% Pt shows an overpotential of -0.1 V, while the CoS/S-rGO/NF electrode loaded with 10% CoS/S-rGO shows an overpotential of -0.155 V. It was shown that the difference was only 0.055 V. The above difference of 0.055 V may have occurred because CoS/S-rGO does not grow well in NF. Therefore, when about 20% CoS/S-rGO is stably loaded into NF, it can be expected that similar performance to Pt/NF will be observed. Moreover, chronopotentiometry shows that the CoS/S-rGO/NF electrode exhibits an overpotential that surprisingly matches the initial overpotential for up to 35,000 s, demonstrating excellent durability of the electrode.

Time-dependent data obtained for the Example 7 electrode over 10 days at 100 mA/cm ² in alkaline and acidic electrolytes are shown in Figures S6 and S7. Figure S6 showed that the overpotential slightly increased from -0.24 V to -0.23 V in the initial stage of the reaction in alkaline electrolyte. However, it returned to its original value after two days, and only negligible changes were observed after that. The overpotential was still the same as the initial value of -0.23 V even after 10 days. In addition, it was observed that gaseous hydrogen and oxygen are strongly generated in the electrode, so the electrode of Example 7 can exhibit excellent durability even when long-term exposure is carried out in strongly alkaline conditions. On the other hand, the initial overpotential of the acidic electrolyte solution is -0.217 V, and does not change over time, so the electrode of Example 7 is stable even when exposed to strong acidic conditions for a long period of time, ensuring excellent durability. Therefore, it can be confirmed that the electrode of Example 7 is very durable in both alkaline and acidic electrolytes.

Claims (18)

Reduced graphene oxide (rGO) doped with sulfur element (S); and
A catalyst for dual-function hydrogen generation comprising: cobalt sulfide (CoS) particles attached to the rGO. In claim 1,
A dual-functional catalyst for hydrogen generation, characterized in that the average XRD crystal lattice distance of rGO doped with the sulfur element (S) is 3.2 to 3.5 Å. In claim 1,
A dual-function catalyst for hydrogen generation wherein the CoS particles have an average diameter of 0.1 ㎛ to 1 ㎛. In claim 3,
In the CoS particles, the Co ³⁺ :Co ²⁺ molar ratio is 1: (1.8 to 2.2). In claim 1,
The catalyst is a dual-function catalyst for hydrogen generation, characterized in that it contains 98 to 99% by weight of CoS particles and 1 to 2% by weight of rGO doped with elemental sulfur (S) based on the total weight of the catalyst. Base substrate; and
A dual-function electrode for hydrogen generation comprising the catalyst according to any one of claims 1 to 5. In claim 6,
A dual-function electrode for hydrogen generation, characterized in that the base material is at least one selected from the group consisting of carbon paper, carbon felt, carbon black, activated carbon, carbon nanofiber, nickel foam, and copper foam. In claim 6,
The electrode is a dual-function electrode for hydrogen generation, characterized in that it has activation of hydrogen evolution reaction (HER) in an alkaline electrolyte and an acidic electrolyte. In claim 8,
The alkaline electrolyte is at least one selected from the group consisting of KOH, NaOH and CsOH, and the acidic electrolyte is at least one selected from the group consisting of H ₂ SO ₄ , HCl, HNO ₃ and HClO ₄ Dual function, characterized in that Electrode for hydrogen generation. In claim 6,
The Tafel slope of the electrode is 80 mV/decade to 100 mV when measured in a 1.0 M aqueous KOH solution or a 0.5 M aqueous H ₂ SO ₄ wavenumber range of 2000 to 300 cm ^-1 at an excitation wavelength of 532 nm. A dual-function electrode for hydrogen generation, characterized in that /decade. In claim 6,
A dual-function electrode for hydrogen generation, characterized in that the charge transfer resistance (R _ct ) value of the electrode is 10 Ω to 20 Ω. A first step of dispersing graphene oxide (GO) in ethylene glycol (EG) and preparing a GO dispersion;
A second step of preparing S-rGO powder by adding a reducing agent to the GO dispersion and stirring it;
A third step of mixing the S-rGO powder with distilled water and stirring it to prepare an S-rGO dispersion solution;
A fourth step of preparing a first mixed solution by adding a cobalt precursor and a sulfur precursor to the S-rGO dispersion solution and stirring;
A fifth step of preparing a second mixed solution by immersing carbon paper (CP) in the first mixed solution and ultrasonicating it; and
A sixth step of sealing the CP and the second mixed solution and collecting an electrode containing CoS particles (CoS/S-rGO) attached to rGO doped with sulfur element (S) overgrown through hydrothermal synthesis;
A method of manufacturing an electrode comprising. In claim 12,
A method for producing an electrode, characterized in that the concentration of the GO dispersion is 1 mg/mL to 5 mg/mL. In claim 12,
The reducing agent includes sodium sulfide (Na ₂ S), ammonium thiocyanate ([NH ₄ S]CN), thiourea ((NH ₂ ) ₂ CS), and thiourea dioxide; (NH)(NH ₂ )CSO ₂ H) and ethane-1,2-dithiol (1,2-Ethanedithiol; C ₂ H ₄ (SH) ₂ ) An electrode characterized in that any one selected from the group consisting of Manufacturing method. In claim 12,
The cobalt precursor is selected from the group consisting of cobalt chloride (CoCl ₂ ·6H ₂ O), cobalt nitrate (Co(NO ₃ ) ₂ ·6H ₂ O), and cobalt acetate ((CH ₃ COO) ₂ Co·4H ₂ O) A method of manufacturing an electrode, characterized in that one or more types are selected. In claim 15,
A method of manufacturing an electrode, characterized in that the concentration of the cobalt precursor is 0.01mM to 1mM. In claim 12,
The sulfur precursor includes sodium sulfide (Na ₂ S), thiourea (CH ₄ N ₂ S), ammonium thiocyanate (NH ₄ SCN), thioacetamide (C ₂ H ₅ NS), Benzyl disulfide (C ₁₄ H ₁₄ S ₂ ), thiophene (C ₄ H ₄ S), 2,2'-dithiophene (2,2'-dithiophene; C ₈ H ₆ S ₂ ), 1 selected from the group consisting of p-toluenesulfonic acid (C ₇ H ₈ O ₃ S), 2-thiophenemethanol (C ₅ H ₆ OS), and sulfur powder (S powder) A method of manufacturing an electrode, characterized in that there are at least three types. In claim 12,
In the sixth step, the hydrothermal synthesis is performed at a temperature of 150 to 200 ° C for 8 to 10 hours.