ELECTROCATALYST FOR WATER ELECTROLYSIS AND PREPARING METHOD OF THE SAME

Description

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No. 10-2024-0010349, filed on Jan. 23, 2024, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The disclosure relates to an electrocatalyst for water electrolysis and a method of preparing the same, and more particularly to an electrocatalyst for water electrolysis, in which MXene with excellent electrical conductivity and high surface area is introduced as a support, and a method of preparing the same.

Description of the Related Art

A fuel cell is attracting attention as a next-generation energy conversion technology because it has a very high-power generation efficiency of 40 to 80%, low noise, and produces an environmentally friendly reaction by-product of water.

However, to commercialize the fuel cell, it is necessary to efficiently supply reactants, i.e., hydrogen and oxygen.

Currently, the general method of obtaining hydrogen and oxygen is water electrolysis, but PowerGrid-scale freshwater electrolysis has limitations because of imposing a large burden on important water resources. In addition, conventional methods of processing fossil fuels to produce hydrogen has a problem that the reserves of fossil fuels are not infinite.

Accordingly, research has been conducted to provide hydrogen through electrolysis using seawater existing on Earth, but a problem of low stability arises due to anode corrosion caused by chloride anions (about 0.5 M) present in seawater. Accordingly, development of technology to solve this problem has been required.

The water electrolysis is classified into a cation exchange membrane (PEM) method and an anion exchange membrane (AEM) method according to the types of electrolytes.

In the case of AEM water electrolysis, advantages of high price competitiveness due to use of hydrocarbon polymer membranes and non-precious metal-based catalysts of alkaline water electrolysis, and the high operating density and compact design of PEM water electrolysis are implemented in combination. However, the AEM water electrolysis has not reached a commercialization stage.

The existing oxygen evolution reaction (OER) electrode requires stable durability due to relatively slow reaction speed and high overvoltage, which limits the use of non-precious metal-based catalysts. Recently, transition metal phosphides that can stably induce catalytic reactions under acidic and basic conditions with excellent electrical conductivity and high exchange current density are being studied.

However, there is a difficulty in commercializing the transition metal phosphides due to a small active area and self-agglomeration. Accordingly, further research on an electrocatalyst for the water electrolysis, to which the transition metal phosphides are applied, is required.

DOCUMENTS OF RELATED ART

• • (Patent Document 1) Korean Patent No. 10-2499949 (published on Aug. 10, 2022)

SUMMARY OF THE INVENTION

The disclosure has been conceived to solve the aforementioned problems that the conventional electrocatalyst for water electrolysis has poor stability and performance, and an aspect of the disclosure is to provide an electrocatalyst for water electrolysis and a method of preparing the same, in which a MXene with excellent electrical conductivity and high surface area is used as a support for supporting transition metal phosphide, thereby improving performance.

According to an embodiment of the disclosure, an electrocatalyst for water electrolysis includes: a support made of a MXene having a two-dimensional structure, and a transition metal compound located on and heterogeneously bonded to the support.

The transition metal compound may include transition metal phosphide.

The transition metal phosphide may include one or more selected from a group consisting of Ni₂P, Fe₂P, MoP, CoP and WP.

The support may be for a hydrogen evolution reaction (HER) or an oxygen evolution reaction (OER).

The support may be applied simultaneously to a positive electrode and a negative electrode of a water electrolysis cell.

The MXene may include a compound based on the following Chemical Formula 1:

• • where, ‘a’ is an integer from 2 to 4, ‘b’ is an integer from 1 to 3, and ‘X’ is carbon (C) or nitrogen (N).

The support may have a surface modified with one or more functional groups selected from a group consisting of —O, —OH and —F.

According to an embodiment of the disclosure, a method of preparing an electrocatalyst for water electrolysis includes: a precursor solution preparation step of preparing a precursor solution by dissolving a transition metal precursor in an organic solvent, a mixed solution preparation step of supplying a phosphorus precursor to the precursor solution to form a mixed solution, and a MXene injection step of supplying a support solution containing a MXene to the mixed solution to prepare an electrocatalyst for water electrolysis.

The transition metal precursor may include one or more selected from a group consisting of nickel acetate, iron acetate, molybdenum acetate, cobalt acetate, and tungsten acetate.

The phosphorus precursor may include an alkyl phosphine-based material.

The alkyl phosphine may include one or more selected from a group consisting of triethyl phosphine, tributyl phosphine, trioctyl phosphine, triphenyl phosphine, and tricyclohexyl phosphine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart of a preparing method of an electrocatalyst for water electrolysis according to the disclosure.

FIG. 2 shows a conceptual diagram of a preparing method of an electrocatalyst for water electrolysis according to the disclosure.

FIG. 3 shows a graph of temperature change over time in a preparing method of an electrocatalyst for water electrolysis according to the disclosure over time.

FIG. 4a shows a photograph of an electrocatalyst for water electrolysis prepared according to a third embodiment of the disclosure, taken with a transmission electron microscope (TEM) (Scale Bar 500 nm).

FIG. 4b shows a photograph of an electrocatalyst as same as that of FIG. 4a, taken with a transmission electron microscope (Scale Bar 50 nm).

FIG. 4c shows a photograph of an electrocatalyst as same as that of FIG. 4a, taken with a transmission electron microscope (Scale Bar 5 nm).

FIG. 5a shows electron energy loss spectroscopy (EELS) analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure.

FIG. 5b shows electron energy loss spectra of the Ti L_2,3 line measured for Ti L_2,3 position marked in FIG. 5a.

FIG. 5c shows electron energy loss spectra of the C-K line measured for C-K position marked in FIG. 5a.

FIG. 6a shows Raman spectroscopy analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and a comparative example (—O, —OH peaks marked).

FIG. 6b shows X-ray diffraction (XRD) analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and a comparative example.

FIG. 7a shows Fourier-transform infrared (FT-IR) spectroscopy analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 7b shows FT-IR spectra of the line measured for C-C position marked in FIG. 7a.

FIG. 8a shows X-ray photoelectron spectroscopy (XPS) analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 8b shows Ti 2p XPS spectrum of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 8c shows C is XPS spectrum of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 9a shows Co 2p XPS spectrum of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 9b shows P 2p XPS spectrum of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 9c shows 0 is XPS spectrum of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples.

FIG. 10a shows HER (Hydrogen Evolution Reaction) current density versus potential curves occurring in a negative electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 10b shows potential changes at different current density conditions (10 mA/cm², 100 mA/cm²) occurring in a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 10c shows Tafel slope for a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 11a shows Nyquist complex-plane impedance spectra in a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 11b shows electrochemical current density versus potential curves in a negative electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured at first scan and 2,000th scan.

FIG. 11c shows electrochemical current density versus potential curves in a negative electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured when the PH is varied.

FIG. 12a shows current density versus potential curves for water oxidation activity (oxygen evolution reaction, OER) occurring in a positive electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 12b shows overpotential calculated at different current density conditions (10 mA/cm², 100 mA/cm²) in a positive electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 12c shows Tafel slope for a positive electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 13a shows Nyquist complex-plane impedance spectra in a positive electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 13b shows electrochemical current density versus potential curves in a positive electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured at first scan and 2,000th scan.

FIG. 13c shows Tafel slope representing electrochemical active surface measured in a positive electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure when the scan rate varied from 20 mV/s to 120 mV/s.

FIG. 14a shows current density versus potential graphs between a cell for water electrolysis, i.e., a two-electrode system in which the third embodiment is applied to each of the positive electrode and the negative electrode, and a cell for water electrolysis, to which a conventionally known configuration (HER: Pt/C, OER: RuO₂) is applied.

FIG. 14b shows current density versus elapsed operation time graphs of comparison in performance between a cell for water electrolysis, i.e., a two-electrode system in which the third embodiment is applied to each of the positive electrode and the negative electrode, and a cell for water electrolysis, to which a conventionally known configuration (HER: Pt/C, OER: RuO₂) is applied.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the disclosure will be described in detail with reference to the accompanying drawings.

The disclosure may be modified in various ways and have various embodiments, and thus specific embodiments will be illustrated by way of example in the accompanying drawings and specifically described in the detailed description. It should be understood, however, that the drawings and descriptions are not intended to limit the disclosure to the specific embodiments, but cover all modifications, equivalents, and alternatives that fall within the spirit and scope of the disclosure.

Although the terms “first,” “second,” etc. may be used herein to describe various components, such components should not be construed as limited by these terms. These terms are only used to distinguish one component from another. For example, a first component may be termed a second component, and the second component may also be termed the first component, without departing from the scope of the disclosure.

The terms “and/or” may include combinations of a plurality of related described items or any of a plurality of related described items.

When a component is described as being “connected” or “coupled” to another component, it should be understood that the component may be directly connected or joined to another component but intervening components may be present therebetween. However, when a component is described as being “directly connected” or “directly coupled” to another component, it should be understood that there are no intervening components therebetween.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the disclosure. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.

It will be further understood that the terms “include” and/or “have” when used herein specify the presence of stated features, numbers, steps, operations, components, parts, and/or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, operations, components, parts, and/or combinations thereof.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the disclosure pertains. It will be further understood that terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the related art and will not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

In addition, the embodiments set forth herein are provided for more complete descriptions to a person having an ordinary knowledge in the art, and the shape, size, etc. of components in the accompanying drawings may be exaggerated for clarity.

An electrocatalyst for water electrolysis according to a first embodiment of the disclosure includes a support made of a MXene having a two-dimensional structure, and a transition metal compound located on and heterogeneously bonded to the support.

The transition metal compound uses transition metal phosphide, and thus stably induces a catalytic reaction under acidic and basic conditions with excellent electrical conductivity and high exchange current density.

The transition metal phosphide may include one or more selected from the group consisting of Ni₂P, Fe₂P, MoP, CoP and WP, and most preferably cobalt phosphide.

The transition metal phosphide may be prepared by reacting a transition metal precursor and a phosphorus precursor according to conventionally known stoichiometry.

For example, the transition metal precursor may include one or more selected from the group consisting of nickel acetate, iron acetate, molybdenum acetate, cobalt acetate, and tungsten acetate, and most preferably cobalt acetate, which has the best activity compared to other precursors.

The phosphorus precursor may include an alkyl phosphine-based material, and the alkyl phosphine may include one or more selected from the group consisting of triethyl phosphine, tributyl phosphine, trioctyl phosphine, triphenyl phosphine, and tricyclohexyl phosphine, and most preferably trioctyl phosphine (TOP), which has the best activity compared to other precursors.

The MXene is prepared by mixing a strong acid alone with or mixing a strong acid and a fluoride salt simultaneously with a titanium inorganic compound (Ti_aAX_b), which has a layered hexagonal structure called a “MAX phase,” to remove metal (A) from the titanium inorganic compound (Ti_aAX_b).

Here, ‘a’ is an integer from 2 to 4, ‘b’ is an integer from 1 to 3, and ‘X’ may be carbon (C) or nitrogen (N).

The strong acid may include a combination of generally known acid compounds such as hydrochloric acid (HCl), iodic acid (HI), sulfuric acid (H₂SO₄), and nitric acid (HNO₃), but is not limited thereto.

Further, the fluoride salt that is mixed with the strong acid and acts as an etching solution is used to remove the metal (A) from the titanium inorganic compound. For example, the fluoride salt may include lithium fluoride (LiF), sodium fluoride (NaF), magnesium fluoride (MgF₂), strontium fluoride (SrF₂), beryllium fluoride (BeF₂), calcium fluoride (CaF₂), ammonium fluoride (NH₄F), ammonium difluoride (NH₄HF₂), ammonium hexafluoroaluminate ((NH₄)₃AlF₆), or a combination thereof.

For example, the titanium inorganic compound includes one or more selected from the group consisting of Ti₂CdC, Ti₂AlC, Ti₂GaC, Ti₂InC, Ti₂TIC, Ti₂AlN, Ti₂GaN, Ti₂InN, Ti₂GeC, Ti₂SnC, Ti₂PbC, Ti₃AlC₂, Ti₃SiC₂, Ti₃GeC₂, Ti₃SnC₂, Ti₄AlN₃, Ti₄GaC₃, Ti₄SiC₃, and Ti₄GeC₃, and is not limited thereto, but most preferably titanium aluminum carbide (Ti₃AlC₂), which has a superior reaction rate compared to other inorganic compounds.

The support is for a hydrogen evolution reaction or an oxygen evolution reaction to be applied to both positive and negative electrodes for anion exchange membrane (AEM) water electrolysis.

In other words, the support is simultaneously applied to the positive and negative electrodes of a water electrolysis cell, thereby providing excellent performance without using the catalyst, which contains precious metal or precious metal oxide, in the positive electrode and the negative electrode, respectively.

Further, the support that contains the MXene may be applied simultaneously to the positive and negative electrodes of the water electrolysis cell, and maintain catalytic properties because the transition metal compound is formed on and stably bonded to the support.

The MXene includes a compound based on the following Chemical Formula 1, and may include two or more kinds of components according to the Chemical Formula 1.

Here, ‘a’ is an integer from 2 to 4, ‘b’ is an integer from 1 to 3, and ‘X’ is carbon (C) or nitrogen (N).

The surface of the support may be modified with one or more functional groups selected from the group consisting of —O, —OH and —F.

Referring to FIGS. 1 to 2, a method of preparing an electrocatalyst for water electrolysis according to a second embodiment of the disclosure includes a precursor solution preparation step S20 of preparing a precursor solution by dissolving a transition metal precursor in an organic solvent, a mixed solution preparation step S30 of supplying a phosphorus precursor to the precursor solution to form a mixed solution, and a MXene injection step S40 of supplying a support solution containing the MXene to the mixed solution to prepare the electrocatalyst for the water electrolysis.

The transition metal precursor includes one or more selected from the group consisting of nickel acetate, iron acetate, molybdenum acetate, cobalt acetate, and tungsten acetate.

The phosphorus precursor may include an alkyl phosphine-based material.

The alkyl phosphine may include one or more selected from the group consisting of triethyl phosphine, tributyl phosphine, trioctyl phosphine, triphenyl phosphine, and tricyclohexyl phosphine.

Before performing the MXene injection step S40, the support solution containing the MXene is prepared in the following order:

• • 1) a MXene reaction step S11, in which a strong acid alone or a strong acid and a fluoride salt are added to the titanium inorganic compound and stirred in a reactor for 16 to 20 hours at a temperature of 30° C. or higher and 40° C. or lower to prepare the MXene,
  • 2) a MXene washing step S12, in which the MXene prepared in the MXene reaction step S11 undergoes sonification in distilled water (DI Water) and then washed through a centrifugal process, and
  • 3) a MXene drying step S13, in which the MXene washed in the MXene washing step S12 is frozen using liquid nitrogen and then dried under reduced pressure under a vacuum condition to obtain the MXene with reduced impurities.

For example, the titanium inorganic compound includes one or more selected from the group consisting of Ti₂CdC, Ti₂AlC, Ti₂GaC, Ti₂InC, Ti₂TIC, Ti₂AlN, Ti₂GaN, Ti₂InN, Ti₂GeC, Ti₂SnC, Ti₂PbC, Ti₃AlC₂, Ti₃SiC₂, Ti₃GeC₂, Ti₃SnC₂, Ti₄AlN₃, Ti₄GaC₃, Ti₄SiC₃, and Ti₄GeC₃, and is not limited thereto, but most preferably titanium aluminum carbide (Ti₃AlC₂), which has a superior reaction rate compared to other inorganic compounds.

In the MXene reaction step, the fluoride salt that is mixed with the strong acid and acts as an etching solution is used to remove the metal (A) from the titanium inorganic compound. For example, the fluoride salt may include lithium fluoride (LiF), sodium fluoride (NaF), magnesium fluoride (MgF₂), strontium fluoride (SrF₂), beryllium fluoride (BeF₂), calcium fluoride (CaF₂), ammonium fluoride (NH₄F), ammonium difluoride (NH₄HF₂), ammonium hexafluoroaluminate ((NH₄)₃AlF₆), or a combination thereof.

The strong acid may include a combination of generally known acid compounds such as hydrochloric acid (HCl), iodic acid (HI), sulfuric acid (H₂SO₄), and nitric acid (HNO₃), but is not limited thereto.

1. Experimental Example

Below, the results of evaluating and comparing physical properties between the electrocatalysts for the water electrolysis, prepared by the method of preparing the electrocatalysts for the water electrolysis according to the disclosure (embodiments) and the electrocatalysts for the water electrolysis according to the related art (comparative examples) will be described.

(1) Embodiments

First, to prepare the MXene, 1 g of titanium aluminum carbide (Ti₃AlC₂) used as the titanium inorganic compound and 10 mg of lithium fluoride (LiF) used as the fluoride salt were added to 10 mL of hydrochloric acid (HCl), and stirred in a reactor at a speed of 3500 rpm to react for 18 hours at a temperature of 35° C., thereby performing the MXene reaction step S11.

Then, the MXene prepared in the MXene reaction step S11 underwent sonification by applying ultrasonic waves to distilled water (DI Water) for 4 hours, and then centrifuged in a centrifuge at a speed of 2000 rpm for 10 minutes, thereby performing the MXene washing step S12. The MXene washing step 12 of washing the MXene was performed 5 times.

Then, the MXene washed in the MXene washing step S12 was frozen using liquid nitrogen, and then dried for 24 hours at a temperature of 100° C. or higher to 200° C. or lower under a low vacuum condition (100 to 760 Torr) to remove residual moisture, thereby performing the MXene drying step S13. In this way, the MXene preparation step S10 was performed to prepare the MXene (Ti₃C₂).

After performing the MXene preparation step S10, 200 mg of cobalt acetylacetonate (Co(acac)₂) used as the transition metal precursor, 25.6 mL of oleylamine used as an organic ligand, and 18 mL of 1-octadecene used as an organic solvent were mixed in a 100 ml flask, and heated at a temperature of 120° C. for 1 hour under a vacuum condition, thereby performing the precursor solution preparation step S20.

Then, 6 ml of trioctyl phosphine used as the phosphorus precursor was injected into the precursor solution, the temperature was raised up to 340° C. at a rate of about 10° C./min, and reaction was performed for 30 minutes or more and 2 hours or less, thereby performing the mixed solution injection step S30 of preparing the mixed solution that contains transition metal phosphide, e.g., CoP.

Then, the support solution containing 200 mg of MXene, which underwent sonification in 3 mL of 1-octadecene, was supplied to the mixed solution, thereby preparing the transition metal phosphide/MXene composite (CoP@Ti₃C₂) through the MXene injection step S40 of preparing the electrocatalyst for the water electrolysis, which contains a transition metal phosphide/MXene composite.

Referring to FIG. 3, the first to fourth embodiments were respectively prepared with different periods of time (t₂), e.g., 10 minutes, 20 minutes, 30 minutes, and 120 minutes, from the time of starting the reaction between the transition metal precursor and the phosphorus precursor as the temperature was raised from 120° C. to 340° C. (t₁) in the mixed solution preparation step S30 to the time of performing the MXene injection step S40, while keeping a period of time (t₂+t₃) to 2 hours and 30 minutes from the time of starting the reaction between the transition metal precursor and the phosphorus precursor to the time of completing the MXene injection step S40.

(2) First Comparative Example

As a comparative example compared to the foregoing embodiments, the MXene preparation step S10 of sequentially performing the MXene reaction step S11, the MXene washing step S12, and the MXene drying step S13 was performed to prepare the MXene (Ti₃C₂).

The details of each step were the same as those of the third embodiment, and the precursor solution preparation step S20, the mixed solution preparation step S30, and the MXene injection step S40 were not performed.

(3) Second Comparative Example

As a comparative example compared to the foregoing embodiments, the precursor solution preparation step S20 and the mixed solution preparation step S30 were performed to prepare the transition metal phosphide, i.e., cobalt phosphide (CoP).

The details of each step were the same as those of the third embodiment, and the MXene reaction step S11, the MXene washing step S12, the MXene drying step S13, and the MXene injection step S40 were not performed.

2. Physical Property Evaluation

Below, the results of evaluating and comparing the physical properties between the transition metal phosphide/MXene composite, which is the electrocatalyst for the water electrolysis prepared by the method of preparing the electrocatalyst for the water electrolysis according to the disclosure, (embodiments) and the MXene (first comparative example) and between the transition metal phosphide/MXene composite (embodiments) and the transition metal phosphide (second comparative example) will be described.

Electron Microscope

Referring to FIGS. 4a-4c, when the structure of the transition metal phosphide/MXene composite prepared according to the third embodiment (t₂=30 minutes) was photographed at different magnifications (500 nm (FIG. 4a), 50 nm (FIG. 4b), and 5 nm (FIG. 4c)) through a transmission electron microscope (TEM), a nanorod of cobalt phosphide having a one-dimensional nanostructure was formed at the level of tens of nanometers on the MXene having a two-dimensional structure, thereby forming a composite structure.

Electronic Structure

Change in the electronic structure between the transition metal phosphide/MXene composites prepared according to the third embodiment (t₂=30 minutes) and the fourth embodiment (t₂=120 minutes) was checked through electron energy loss spectroscopy (EELS). As shown in FIG. 5a, the full spectrum of the transition metal phosphide/MXene composite (CoP@Ti₃C₂) was checked, and then each Ti L_2,3 line and C-K line was shown in FIG. 5b and FIG. 5c.

The spectrum results of FIG. 5a shows that the shifted peaks (see the marks in FIG. 5a) of titanium and carbon indicate electron transfer from the MXene to the transition metal phosphide (cobalt phosphide) as a chemical bond is formed in the process of forming the transition metal phosphide/MXene composite.

In particular, the degree of electron transfer indicates the extent to which the MXene was involved in an early stage of growing transition metal phosphide, and it is assumed that a strong bond was formed between the transition metal phosphide and MXene surface atoms. Thus, it is judged that the overall stability of the transition metal phosphide/MXene composite is the best.

Change in Chemical Structure

Referring to FIG. 6a and FIG. 6b, change in the chemical bond between the transition metal phosphide/MXene composites prepared according to the third embodiment (t₂=30 minutes) and the fourth embodiment (t₂=120 minutes) was checked through Raman spectroscopy (see FIG. 6a) and X-ray diffraction (XRD) (see FIG. 6b), compared to the first comparative examples.

As shown in FIG. 6a, many surface functional groups are present in the form of —O and —OH in the MXene (comparative example), and combine with transition metal phosphide to form the transition metal phosphide/MXene composites. As the peak intensity corresponding to the surface functional groups (—O and —OH) is weakened, the surface functional groups are reduced and new M-O—X and M-P—O bonds are formed in the process of combining with the transition metal phosphide.

As shown in FIG. 6b, when difference in crystallinity was analyzed through X-ray diffraction spectrums, it was found that the peak indicating the (002) crystal plane around 5° was shifted to the left. This means that an interlayer distance of a 2D MXene is increased as transition metal phosphate combines with the surface functional groups of the MXene.

In addition, the peak indicating the (201) crystal plane of transition metal phosphide (Co₂P) around 40° was commonly found in the third and fourth embodiments, but the peak indicating a (220) crystal plane of R—Co(OH)₂ around 30° was observed only in the third embodiment. This is regarded as a unique change caused by the involvement of the MXene in the early stages of growing cobalt phosphide.

Further, referring to FIG. 7a and FIG. 7b, change in the chemical bond between the transition metal phosphide/MXene composites prepared according to the third embodiment (t₂=30 minutes), the fourth embodiment (t₂=120 minutes) and the comparative examples was checked through Fourier-transform infrared (FT-IR) spectroscopy.

Check of Chemical Bonding State

FIG. 8a show X-ray photoelectron spectroscopy (XPS) analysis graphs of electrocatalysts for water electrolysis prepared according to third and fourth embodiments of the disclosure and comparative examples, and then each Ti 2p line and C is line of the electrocatalysts are shown in FIG. 8b and FIG. 8c.

FIGS. 9a-9c show each XPS Co 2p spectrum, P 2p spectrum, and O 1s spectrum of the electrocatalysts as same as that of FIG. 8a.

The Ti 2p spectrum (see FIG. 8b) and the C 1s spectrum (see FIG. 8c) showed that the peaks of the transition metal phosphate/MXene composites (third and fourth embodiments) were shifted to high binding energy compared to the peaks of the MXene (comparative example). Thus, it was confirmed that electron transfer from the MXene to cobalt phosphide occurred as the transition metal phosphate/MXene composite was formed.

As a corresponding result, the P 2p spectrum (see FIG. 9b) showed that the peak of the transition metal phosphate/MXene composite was shifted to lower binding energy compared to that of the existing CoP. In addition, the Co 2p spectrum (see FIG. 9a) and the O 1s spectrum (see FIG. 9c) showed that a Co—O—P—Ti and Co—P—O—Ti bond was formed in the bond between the MXene and cobalt phosphide.

Reaction in Negative Electrode

FIG. 10a shows HER (Hydrogen Evolution Reaction) current density versus potential curves occurring in a negative electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 10b shows potential changes at different current density conditions (10 mA/cm², 100 mA/cm²) occurring in a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 10c shows Tafel slope for a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 11a shows Nyquist complex-plane impedance spectra in a negative electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 11b shows electrochemical current density versus potential curves in a negative electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured at first scan and 2,000th scan.

FIG. 11c shows electrochemical current density versus potential curves in a negative electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured when the PH is varied.

This measurement was conducted using a three-electrode system including Hg/HgO (1M NaOH) and Pt wires as reference and counter electrodes, respectively, in 1M KOH (pH-14) electrolyte, and detailed descriptions of other specific methods will be omitted because they are well known to those skilled in the art.

The first embodiment (t₂=10 minutes), the second embodiment (t₂=20 minutes), and the third embodiment (t₂=30 minutes) were samples of comparison groups at the growth beginning of cobalt phosphide, and the fourth embodiment (t₂=120 minutes) was a sample where the MXene was injected at the growth completion of cobalt phosphide. Among the samples, a sample showing the best performance was the third embodiment (t₂=30 minutes).

The reaction current by each potential for the HER is shown in the current density-potential (J-V) graph of FIG. 10a, and quantified to show the catalyst activity for each sample through the overvoltage by current density of FIG. 10b and the Tafel slope of FIG. 10c.

In the case of the third embodiment 3 that showed the best performance, the overvoltage was 118 mV at a current density of 10 mA/cm² and was 189 mV at a current density of 100 mA/cm² as shown in FIGS. 10a and 10b, which results from the excellent charge transfer and charge behavior confirmed through the Tafel slope of FIG. 10c and the electrochemical impedance spectroscopy in FIG. 11a.

Further, the results of current density-potential (J-V) measurement after repeated 2000 times in FIG. 11b show that the third embodiment has the excellent stability as the negative electrode, and the results of measurement for each electrolyte pH in FIG. 11c show that Ag/AgCl(3M NaCl) and Pt wires respectively measured as the reference and counter electrodes in 0.1M phosphate buffer (pH-7) and 0.5M H₂SO₄ (pH-0) electrolytes are applicable to a full range of liquid without being limited to acidic conditions.

Reaction in Positive Electrode

FIG. 12a shows current density versus potential curves for water oxidation activity (oxygen evolution reaction, OER) occurring in a positive electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 12b shows overpotential calculated at different current density conditions (10 mA/cm², 100 mA/cm²) in a positive electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 12c shows Tafel slope for a positive electrode to which electrocatalysts for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 13a shows Nyquist complex-plane impedance spectra in a positive electrode to which electrocatalysts (transition metal phosphide/MXene composite (CoP@Ti₃C₂)) for water electrolysis prepared according to first to fourth embodiments of the disclosure and comparative examples are applied.

FIG. 13b shows electrochemical current density versus potential curves in a positive electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure measured at first scan and 2,000th scan.

FIG. 13c shows Tafel slope representing electrochemical active surface measured in a positive electrode to which electrocatalysts for water electrolysis prepared according to third embodiment of the disclosure when the scan rate varied from 20 mV/s to 120 mV/s.

This measurement was conducted by a three-electrode system including Hg/HgO (1M NaOH) and Pt wires as reference and counter electrodes, respectively, in 1M KOH (pH-14) electrolyte, and detailed descriptions of other specific methods will be omitted because they are well known to those skilled in the prior art.

The first embodiment (t₂=10 minutes), the second embodiment (t₂=20 minutes), and the third embodiment (t₂=30 minutes) were samples of comparison groups at the growth beginning of cobalt phosphide, and the fourth embodiment (t₂=120 minutes) was a sample where the MXene was injected at the growth completion of cobalt phosphide. Among the samples, a sample showing the best performance was the third embodiment (t₂=30 minutes).

The reaction current by each potential for the OER is shown in the current density-potential (J-V) graph of FIG. 12a, and quantified to show the catalyst activity for each sample through the overvoltage by current density of FIG. 12b and the Tafel slope of FIG. 12c.

In the case of the third embodiment that showed the best performance, the overvoltage was 250 mV at a current density of 10 mA/cm² and was 430 mV at a current density of 100 mA/cm² as shown in FIGS. 12a and 12b, which results from the excellent charge transfer and charge behavior confirmed through the Tafel slope of FIG. 12c and the electrochemical impedance spectroscopy in FIG. 13a.

Further, the results of 2000 repeated measurements in FIG. 13b show that the third embodiment has the excellent stability as the positive electrode, and the excellent electrochemical active area in FIG. 13c show high hydroxylation properties.

Cell Performance for Water Electrolysis

FIG. 14a shows current density versus potential graphs between a cell for water electrolysis, i.e., a two-electrode system in which the third embodiment is applied to each of the positive electrode and the negative electrode, and a cell for water electrolysis, to which a conventionally known configuration (HER: Pt/C, OER: RuO₂) is applied.

FIG. 14b shows current density versus elapsed operation time graphs of comparison in performance between a cell for water electrolysis, i.e., a two-electrode system in which the third embodiment is applied to each of the positive electrode and the negative electrode, and a cell for water electrolysis, to which a conventionally known configuration (HER: Pt/C, OER: RuO₂) is applied.

The current density-cell potential graph in FIG. 14a shows that the cell for the water electrolysis according to the third embodiment has lower potential than the cell for the water electrolysis according to the conventional commercial catalyst under the same current density, and is thus driven relatively effectively. Further, the long-term operation stability measurement in FIG. 14b shows that the cell for the water electrolysis according to the third embodiment exhibits improved operation stability compared to the cell for the water electrolysis according to the conventional commercial catalyst.

As described above, there are provided the electrocatalyst for the water electrolysis according to the disclosure and the method of preparing the same, which have effects on increasing electrochemical activity by improving the operation stability and increasing the surface area compared to the conventional commercial catalysts.

Although specific embodiments of the disclosure have been described above in detail, the embodiments are for illustrative purposes only and do not limit the disclosure thereto, and various changes and modifications can be made therein by a person having ordinary knowledge in the art without departing from the scope of the disclosure.

Any simple modification or change in the embodiments falls within the scope of the disclosure, and the specific protection scope of the disclosure will become clear by the appended claims.

DESCRIPTION OF REFERENCE NUMERALS

• • S10: MXene preparation step
  • S20: precursor solution preparation step
  • S30: mixed solution preparation step
  • S40: MXene injection step