WO2020080637A1 - Metal-organic framework-based water electrolysis catalyst derived from prussian blue analog comprising three transition metals and comprising size-controlled pores, and method of preparing same - Google Patents

Abstract

Disclosed is a metal-organic framework-based water electrolysis catalyst prepared from a Prussian blue analog. The metal-organic framework-based water electrolysis catalyst comprises, as transition metals, Ni having an oxidation number of 2, Co having an oxidation number of 2, Co having an oxidation number of 3, Fe having an oxidation number of 2, and Fe having an oxidation number of 3, and comprises a coordination compound in which [Co(CN)₆]^3- complex ions are bonded to Ni^{2 +}, a coordination compound in which [Fe(CN)₆]^3- complex ions are bonded to Ni^{2 +}, and NiS produced by an ion-exchange reaction of the [Co(CN)₆]^3- complex ions and S^2- ions.

Description

A metal-organic structure-based water electrolytic catalyst comprising a size-controlled pore derived from a Prussian blue analog containing three transition metals and a method for manufacturing the same.

The present invention relates to a metal-organic structure-based water electrolytic catalyst derived from a ternary Ni-Co-Fe prussian blue analog and having a controlled pore size, and a method for manufacturing the same.

Hydrogen production due to electrochemical water electrolysis reaction is a technology that is spotlighted as clean energy. However, for the hydrogen evolution reaction (Hydrogen Evolution Reaction, HER) and the oxygen evolution reaction (Oxygen Evolution Reaction, OER) in the water electrolysis reaction, an efficient electrode catalyst for overcoming the activation energy is required. Particularly, in the water electrolysis reaction, oxidation occurs at the anode and oxygen is generated. For the OER, a high overvoltage of the anode is required, thereby increasing the hydrogen production cost. In order to solve this, a catalyst capable of lowering the overvoltage to the OER has been studied. Platinum catalyst was known to be the most efficient catalyst in HER and iridium catalyst in OER. However, platinum and iridium are difficult to commercialize due to the scarcity of materials and high price. Therefore, there is a need for a non-noble metal electrode catalyst that can replace the noble metal catalyst.

Metal organic frameworks (MOFs) using transition metals have high porosity and structural rigidity, and are in the spotlight as precursors for developing functional catalysts. Therefore, a non-precious metal catalyst that can replace a precious metal catalyst in a hydrolysis reaction using MOF has been studied. For example, MOF electrocatalysts using a ZIF (Zeolitic Imidazolate Framework) framework are known. It is also known that MOF catalysts containing Co-N-C species exhibit efficient OER and ORR activity. Also known is an OER catalyst based on a carbon-coated nickel phosphides (NiP) made from Ni-based PBA (Prussian Blue analog). However, bifunctional MOF catalysts that are active against both HER and OER have not been developed yet.

The metal-organic structure-based water electrolytic catalyst according to an aspect of the present invention can replace an expensive platinum catalyst as a non-precious metal catalyst exhibiting low overvoltage in a hydrogen production reaction.

Metal organic structure-based water electrolytic catalyst according to another aspect of the present invention can replace the expensive iridium catalyst as a non-precious metal catalyst exhibiting a low overvoltage in the oxygen production reaction.

The metal-organic structure-based water electrolytic catalyst according to another aspect of the present invention can replace an expensive precious metal catalyst as a non-precious metal catalyst that exhibits a low overvoltage in a hydrogen production reaction and a low overvoltage in an oxygen production reaction.

One aspect of the present invention is represented by the formula of Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} wherein X is prepared from a Prussian blue analog (PBA) of 0.1 to 0.9, and the oxidation number 2 as a transition metal. of Ni, Co oxidation states 2, 3 oxidation number of Co, Fe in oxidation state 2 and 3 comprises an oxidation number of Fe, and [Co (CN) _6] ^3- complex ion and Ni ^{+ 2} binding coordination compound, [Fe ( CN) _6] ^3- complex ion and Ni ^{2 +} fitted with a coordination compound, and the [Co (CN) _6] metal organic framework comprising a ^3-complex ion and the NiS created in the ion exchange reaction of the S ^2- ion A sieve-based water electrolytic catalyst is provided.

Another aspect of the present invention is represented by the formula of Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} wherein X is 0.1 to 0.9 to prepare a Prussian Blue Analog (PBA), the Prussian Blue [Co (CN) ₆ ] of the analog ^3- ion exchange reaction of the complex ion and S ^2- ion, washing and drying the intermediate product subjected to the ion exchange reaction, crushing the dried intermediate product, and the It provides a method for manufacturing a metal-organic framework-based water electrolytic catalyst comprising the step of heat-treating the crushed intermediate product.

The metal-organic framework-based water electrolytic catalyst according to an aspect of the present invention may have a low overvoltage for HER by containing doped nitrogen.

The metal organic skeleton-based water electrolytic catalyst according to another aspect of the present invention may have a low overvoltage for the OER by lowering the number of outermost electrons of the transition metal.

The metal-organic framework-based water electrolytic catalyst according to another aspect of the present invention may have high strength and high porosity.

Metal organic skeleton-based water electrolytic catalyst according to another aspect of the present invention can be produced in a relatively simple method.

The metal-organic framework-based water electrolytic catalyst according to another aspect of the present invention may simultaneously have a low overvoltage for HER and a low overvoltage for OER.

1 shows the crystal structure of a Prussian blue analog.

2 shows SEM images of Prussian blue analogs.

3 shows an SEM image of a metal organic structure.

4 shows a TEM image of the metal organic structure.

5 shows the XRD pattern of the metal organic structure.

Figure 6 shows the XRD pattern of the Prussian blue analogs.

7 shows an SDS image scanned by EDS line of a metal organic structure.

8 shows the EDS line scan results of the center and edge of NC-MOF particles.

9 shows the EDS line scan results of the center and edge of the particles of NF-MOF.

Figure 10 shows the results of the EDS line scan of the center and edge of the particles of the NCF-MOF.

11 shows SAED patterns of metal organic structures.

12 shows DFTEM images and elemental maps of metal organic structures.

13 shows the isotherm of the adsorption (absorption) and desorption (desorption) of nitrogen of NCF-MOF and NF-MOF.

14 shows the pore size distribution of NCF-MOF and NF-MOF. The total volume of the MOF pores can be determined from the weight of the material used.

15 shows the overall XPS analysis results of the metal organic structure.

Figure 16 shows the high-resolution XPS spectrum of Ni 2p and S 2p of NC-MOF, Ni 2p and Fe 2p of NF-MOF.

FIG. 17 shows Ni 2p (A), Fe 2p (B), Co 2p (C), S 2p (D), N 1s (F) of NCF-MOF, and N 1s (E) of NF-MOF.

18 shows the HER curve and the OER polarization curve of the GCE electrode carrying the metal organic skeleton.

FIG. 19 shows the HER polarization curve and HER Tapel Slope of the Nickel Foam Electrode (NFE) carrying a metal-organic framework.

20 shows an OER polarization curve and an OER Tapel Slope of a Nickel Foam Electrode (NFE) carrying a metal organic skeleton.

FIG. 21 shows the HER and OER polarization curves of a nickel foam electrode carrying nothing and a metal organic skeleton.

22 shows the change in overvoltage in a HER or OER by a chronopotentiometry method.

Figure 23 shows the OER polarization curves of NCF-MOF and Ir / C before and after 1000 CV cycles.

FIG. 24 shows the TEM images of the NCF-MOF catalyst ink (including carbon black) before (A), after the endurance test for A's OER (B), and after the endurance test for HER (C). . Also shown are TEM images of the NCF-MOF catalyst ink (including carbon black) before the electrochemical test (D), after the endurance test for A's OER (E) and after the endurance test for HER (F).

In this specification, water electrolysis refers to the electrochemical decomposition of water, and specifically refers to the production of hydrogen and oxygen through a hydrogen generating reaction and an oxygen generating reaction.

In the present specification, the metal organic framework refers to a metal organic structure (MOF) synthesized by using a complex ion and a transition compound in which a transition metal and an organic ligand are combined as a precursor.

Throughout this specification, PBA (Prussian Blue Analog) is a precursor for preparing MOF-based electrolytic catalysts. The PBA may be represented by the formula of M _a ^II [M _b ^III (CN) ₆ ] _{c ·} H ₂ O. The M ^II means a divalent transition metal cation, and the M ^III means a trivalent transition metal cation. The PBA may have a nano-cube shape. The crystal structure of the PBA may be a face-centered cubic structure (FCC).

Metal organic skeleton-based water electrolytic catalyst according to an aspect of the present invention is represented by the formula of Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} and X is 0.1 to 0.9 Prussian blue analog (PBA It is prepared from), and includes as a transition metal Ni of 2 oxidation, Co of 2 oxidation, Co of 3 oxidation, Fe of 2 oxidation and Fe of 3 oxidation, [Co (CN) ₆ ] ^3- complex ion and Ni ^{2 +} Coordinated Compound, [Fe (CN) ₆ ] ^3- Complex Ion and Ni ^{2 +} Coordinated Compound, and [Co (CN) ₆ ] ^3- Ion Exchange Reaction of Complex Ion and S ^2- Ion It contains NiS.

The metal-organic framework based water electrolytic catalyst is prepared from PBA represented by the formula Ni ₃ [Fe _X Co _1-X (CN) ₆ ] ₂ , Ni ₃ [Co (CN) ₆ ] And Ni ₃ [Fe (CN) ₆ ], which may have an effect that the metal-organic framework prepared from PBA represented by the formula does not have. For example, the metal-organic framework-based water electrolytic catalyst may have a low overvoltage for HER by containing doped nitrogen. Alternatively, the metal-organic framework-based water electrolytic catalyst may have a low overvoltage for the OER by lowering the number of outermost electrons of the transition metal. Alternatively, the metal-organic framework-based water electrolytic catalyst may have high porosity while having appropriate strength. Excellent effects as a water electrolytic catalyst of a metal-organic framework prepared from PBA having both Co and Fe will be described in detail later.

The X is 0.1 to 0.9, 0.2 to 0.9, 0.3 to 0.9, 0.4 to 0.9, 0.5 to 0.9, 0.6 to 0.9, 0.1 to 0.8, 0.2 to 0.8, 0.3 to 0.8, 0.4 to 0.8, 0.5 to 0.8, 0.6 to 0.8 , 0.1 to 0.7, 0.2 to 0.7, 0.3 to 0.7, 0.4 to 0.7, 0.5 to 0.7, 0.1 to 0.6, 0.2 to 0.6, 0.3 to 0.6, 0.4 to 0.6, 0.5 to 0.6, preferably 0.3 to 0.7 It may be, more preferably from 0.4 to 0.6.

When the above X is too small _{Ni 3 [Fe (CN) 6} ] less the content of the _{second over-Ni 3 [Co (CN) 6} ] the content of the _two is too large _{^{[Co (CN) - 6]}} 3 - ions and S ^{2 -The} ion-exchange reaction of ions may occur excessively, and thus the porosity of the metal-organic framework may be excessively large, the density may be excessively low, and strength or durability may be weakened. Wherein X is too large _{Ni 3 [Fe (CN) 6} ] ₂ is too large, the content of _{Ni 3 [Co (CN) 6} ] ₂ is too small the amount of _{^{[Co (CN) - 6]}} 3 - and S ^2- ion The ion exchange reaction of ions may occur too little, which may cause the porosity of the metal-organic framework to be too small, and it may be difficult to have a specific surface area as a suitable water electrolytic catalyst.

The preparation is represented by the formula of Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} and wherein X is 0.1 to 0.9, preparing a Prussian Blue Analog (PBA), [ Co (CN) ₆ ] ^3- Completion ion and S ^2- ion ion exchange reaction, washing and drying the intermediate product subjected to the ion exchange reaction, grinding the dried intermediate product, and grinding the intermediate product It may be manufactured by a method of manufacturing a metal-organic skeleton-based water electrolytic catalyst comprising the step of heat-treating the product.

The coordination compound in which the [Co (CN) ₆ ] ^3- complex ion and Ni ^{2 +} are combined may be Ni ₃ [Co (CN) ₆ ] ₂ . The coordination compound in which the [Fe (CN) ₆ ] ^3- complex ion and Ni ^{2 +} are combined may be Ni ₃ [Fe (CN) ₆ ] ₂ .

The metal-organic framework-based water electrolytic catalyst has a size of 150 to 210 nm, 160 to 210 nm, 170 to 210 nm, 180 to 210 nm, 150 to 200 nm, 160 to 200 nm, 170 to 200 nm, 180 to 200 nm, 150 to 190 nm, 160 to It may be 190 nm, 170 to 190 nm, 180 to 190 nm, 150 to 180 nm, 160 to 180 nm, 170 to 180 nm, and preferably 170 to 190 nm.

The metal-organic skeleton-based water electrolytic catalyst may have a rough shape in which the external shape observed by SEM is maintained while maintaining a general nano-cube shape.

The content of Fe 3 as the oxidation water may be higher than the content of Fe as the oxidation number 2 based on XPS analysis.

The content of Fe, the oxidation number 2, may be 40 to 46% by weight based on the total content of Fe.

The metal-organic framework-based water electrolytic catalyst may further include doped nitrogen. At least some of the doped nitrogen may be nitrogen derived from CN ^- produced by the ion exchange reaction. More specifically, [Co (CN) ₆ ] CN ^- can be generated by ion exchange reaction of ^3- ions and S ^2- ions, and the generated CN ^- is by Ni ₃ [Fe (CN) ₆ ] ² CNO ^- can be oxidized to the CNO ^- may be the NH ₃ produced is further decomposed. The NH ₃ may be doped into a metal organic skeleton. That is, in order for the doped nitrogen to be included in the metal organic skeleton-based water electrolytic catalyst, Ni ₃ [Fe (CN) ₆ ] ² And Ni ₃ [Co (CN) ₆ ] ² , all of which contain PBA to be subjected to ion exchange reaction with S ² . The doped nitrogen is 70 to 90%, 75 to 90%, 80 to 90%, 85 to 90%, 70 to 85%, 75 to 85 in CN of [Co (CN) ₆ ] ^3- ions reacted with ion exchange %, Or 80 to 85%, preferably 83 to 90%, or 84 to 89%.

At least some of the doped nitrogen may be pyridine nitrogen. The pyridine nitrogen is formed from a carbon-nitrogen-carbon bond formed by nitrogen doping during a carbon-carbon bond in a cyclic structure in which six carbons are bonded in a structure such as a graphite layer. Refers to the nitrogen atom exposure.

The metal-organic framework-based water electrolytic catalyst containing doped nitrogen may include a graphitic carbon network structure derived from a cyano group at least in part. The pyridine nitrogen may be located in the carbon structure. The carbon structure may include a graphite layer in part.

The doped nitrogen improves electrical conductivity and increases the reactivity of HER and OER catalysts. The doped nitrogen can reduce the bandgap so that the current flows more easily and the overvoltage in the HER can be reduced.

The metal-organic framework-based water electrolysis catalyst may include a carbon structure derived from a cyano group, and the carbon structure may include nitrogen doped from the cyano group. The cyano group may be a cyano group produced by an ion exchange reaction of [Co (CN) ₆ ] ^3- ions and S ^2- ions.

The metal-organic framework-based water electrolytic catalyst has a specific surface area (BET) of 30 to 60 m ^{2 ·} g ^-1 , 40 to 60 m ^{2 ·} g ^-1 , 45 to 60 m ^{2 ·} g ^-1 , 30 to 55 m ^{2 ·} g ^-1 , 40 to 55 m ^{2 ·} g ^-1 , or 45 to 55 m ^{2 ·} g ^-1 , and preferably 47 to 53 m ^{2 ·} g ^-1 .

The metal-organic skeleton-based water electrolytic catalyst may include pores.

At least some of the pores may be mesopores.

According to the recommendation of IUPAC, pores having a diameter of 2 nm to 50 nm are referred to as mesopores, and pores having a diameter of 2 nm or less are referred to as micropore. As used herein, the terms "mesopores" and "micropores" mean pores according to the recommendations of the IUPAC, and the mesopores mean pores with a diameter of 2 nm to 50 nm.

The metal organic skeleton-based water electrolytic catalyst may exhibit hysteresis loops in a certain pressure range by analyzing the N ₂ adsorption and desorption isotherms. The pressure range in which the hysteresis loop appears is 0.5 to 1.1, 0.6 to 1.1, 0.7 to 1.1, 0.8 to 1.1, 0.5 to 1.0, 0.6 to 1.0, 0.7 to 1.0, 0.8 to 1.0, 0.5 to 0.9, 0.6 to 0.9, 0.7 to It may be 0.9, 0.5 to 0.8, 0.6 to 0.8, preferably 0.5 to 1.0, 0.6 to 1.0, 0.6 to 1.1, and more preferably 0.6 to 1.0. If the hysteresis loop is shown in the above pressure range, it can be seen that it contains mesopores.

At least some of the pores may be pores generated by an ion exchange reaction of [Co (CN) ₆ ] ^3- ions and S ^2- ions.

The volume of the pores is 0.15 to 0.35 cc · g ^-1 , 0.20 to 0.35 cc · g ^-1 , 0.25 to 0.35 cc · g ^-1 , 0.15 to 0.32 cc · g ^-1 , 0.20 to 0.32 cc · g ^-1 , It may be 0.25 to 0.32 cc · g ^-1 , 0.15 to 0.30 cc · g ^-1 , 0.20 to 0.30 cc · g ^-1 , 0.25 to 0.30 cc · g ^-1 . The volume of the pores is an analysis of the volume of the pores per gram by the Barrett-Joyner-Halenda (BJH) method.

The sum of the number of outermost electrons of Fe of 2 oxidation number, Fe of 3 oxidation number, Co of 2 oxidation number, and Co of 3 oxidation number is 5.3 to 5.6, 5.4 to 5.6, 5.3 to 5.5, 5.4 to 5.5, 5.3 to 5.5, or 5.4 to 5.5. The method of obtaining the sum of the number of outermost electrons is well known to a person skilled in the art, and the method is also described in Experimental Example 9 below. The lower the sum of the number of outermost electrons, the smaller the adsorption energy of the oxygen intermediate, so that the overvoltage to the OER decreases. Referring to Example 9 below, when the number of outermost electrons was 5.49, an overvoltage of 20 mV lower than that of the Ir / C catalyst, which is a catalyst known to be the best in OER, was exhibited.

Ni combined with S of the NiS is 1 to 10%, 2 to 10%, 3 to 10%, 4 to 10%, 5 to 10%, 6 to 10%, 7 to 10%, 8 to 10 based on total Ni 10%, 9-10%, 1-9%, 2-9%, 3-9%, 4-9%, 5-9%, 6-9%, 7-9%, 8-9%, 1- 8%, 2-8%, 3-8%, 4-8%, 5-8%, 6-8%, 7-8%, 1-7%, 2-7%, 3-7%, 4-8 7%, 5-7%, 6-7%, 1-6%, 2-6%, 3-6%, 4-6%, or 5-6%, preferably 3-7 %, And more preferably 4 to 6%.

The NiS may be at least partially amorphous. It can be confirmed that the amorphous NiS does not exhibit a characteristic NiS pattern in XRD pattern analysis.

The NiS may be formed by ion exchange reaction of [Co (CN) ₆ ] ^3- ions and S ^2- ions. The ratio of NiS may be smaller than the decrease in the ratio of Co of the metal organic structure after the ion exchange reaction in the ratio of Co of the Prussian blue analog.

The metal organic skeleton-based water electrolytic catalyst is (200) plane, (220) plane, (400) plane, (420) plane, (422) plane, (440) plane, (600) plane, (XRD) 620) The peak of the surface may be observed. More specifically, the metal-organic framework-based water electrolytic catalyst has a peak of 2θ from 12.5 ° to 14.5 ° in the (200) plane, a peak of 2θ to 23.5 ° to 25.5 ° in the (220) plane, and 400 in the XRD measurement. The surface peak is 2θ 34˚ to 36˚, the (420) plane peak is 2θ 38˚ to 40˚, the (422) plane peak is 2θ 42˚ to 44˚, and the (440) plane peak is 2θ 49.5˚ to 51.5 ˚, the (600) plane peak may be observed at 2θ 53˚ to 55˚, and the (620) plane peak at 2θ 55 to 57˚. The (200) plane peak may have a greater intensity than the (220) plane peak. The (400) plane peak may have a greater intensity than the (420) plane peak. The (400) plane peak may have a greater intensity than the (422) plane peak. The peak is JCPDS card No. It may be similar to 89-3738.

The precursor of the metal-organic framework-based water electrolysis catalyst is (200) plane, (220) plane, (400) plane, (420) plane, (422) plane, (440) plane, (600) plane in XRD measurement. , The peak of the (620) plane may be observed. Specifically, the precursor of the metal-organic framework-based water electrolytic catalyst is XRD measurement in the (200) plane peak is 2θ 12.5˚ to 14.5˚, the (220) plane peak is 2θ 23.5˚ to 25.5˚, the (400) ) The plane peak is 2θ 34˚ to 36˚, the (420) plane peak is 2θ 38˚ to 40˚, the (422) plane peak is 2θ 42˚ to 44˚, and the (440) plane peak is 2θ 49.5˚ to 51.5˚, the (600) plane peak may be observed at 2θ 53˚ to 55˚, and the (620) plane peak at 2θ 55 to 57˚.

The peak observed in the XRD measurement of the metal-organic framework-based water electrolytic catalyst may be the same peak observed in the XRD measurement of the precursor of the metal-organic framework-based water electrolysis catalyst.

Another aspect of the present invention provides a catalyst ink comprising the metal-organic framework-based water electrolytic catalyst. The catalyst ink may be prepared by mixing a metal organic skeleton-based water electrolytic catalyst with a solution. The solution may be a Nafion solution. The solution may further include Vulcan Carbon. The catalyst ink uses Nafion solution as a solvent and contains 1 to 3 mg / ml, or 1.5 to 2. 5 mg / ml of a metal organic skeleton based water electrolytic catalyst, and 1 to 3 mg of Vulcan Carbon (XC-72, VC) / ml or 1.5 to 2.5 mg / ml. The catalyst ink may be prepared as a uniform suspension by dissolving a metal-organic framework-based water electrolytic catalyst and sonicating for 40 to 70 minutes, 50 to 70 minutes, or 55 to 65 minutes.

Another aspect of the present invention provides a nickel foam electrode (Nickel Foam Electrode, hereinafter referred to as NFE) for the water electrolysis comprising the metal-organic framework-based water electrolysis catalyst. The nickel-electrode for water electrolysis can be prepared by mixing the metal-organic framework-based water electrolysis catalyst in a solution and drop casting it into NFE. The solution may be a Nafion solution. The solution may further include Vulcan Carbon.

Method for producing a metal-organic framework-based electrolytic catalyst according to another aspect of the present invention is represented by the formula of Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _2- and X is 0.1 to 0.9 Prussian Blue Preparing an analog (PBA), [Co (CN) ₆ ] of the Prussian blue analog is ion-exchanged with ^{3 -} ion ions and S ^2- ions, and the intermediate product washed with the ion-exchange reaction is washed and dried. Step, crushing the dried intermediate product and heat-treating the crushed intermediate product.

The X is 0.1 to 0.9, 0.2 to 0.9, 0.3 to 0.9, 0.4 to 0.9, 0.5 to 0.9, 0.1 to 0.8, 0.2 to 0.8, 0.3 to 0.8, 0.4 to 0.8, 0.5 to 0.8, 0.1 to 0.7, 0.2 to 0.7 , 0.3 to 0.7, 0.4 to 0.7, 0.5 to 0.7, 0.1 to 0.6, 0.2 to 0.6, 0.3 to 0.6, 0.4 to 0.6, 0.5 to 0.6, 0.1 to 0.5, 0.2 to 0.5, 0.3 to 0.5, 0.4 to 0.5 And preferably 0.3 to 0.7, and more preferably 0.4 to 0.6.

When the X is increased, the porosity of the metal-organic framework-based water electrolytic catalyst may decrease, and when the X is decreased, the porosity of the metal-organic framework-based water electrolytic catalyst may increase.

Increasing the X may increase the density of the metal-organic framework-based water electrolytic catalyst, and decreasing the X may decrease the density of the metal-organic framework-based water electrolytic catalyst.

The step of preparing the Prussian blue analog (PBA) comprises dissolving nickel nitrate and sodium citrate in water to prepare a first solution, potassium hexacyanoferate (III) in the above X ratio, potassium hexa Preparing a second solution by dissolving cyanocobalt (III) in water at the 1-X ratio, preparing a third solution by mixing the first solution and the second solution, and the third solution. And coprecipitation to obtain a Prussian Blue Analog (PBA).

The water may be ultrapure water in which almost no ions are present, and in detail, it may be tertiary water, and more specifically, Mili-Q water.

It is known that the rate of crystal growth of a metal-organic framework is relatively fast compared to the rate of nucleation. Therefore, a crystal growth rate inhibitor is required to obtain a small and even PBA. The citrate ion of the sodium citrate inhibits crystal growth and prevents formation of excessively large particles during the coprecipitation process.

The ion exchange reaction may be a step of mixing the Prussian blue analog with Na ₂ S and subjecting the mixture to a hydrothermal reaction. The hydrothermal reaction may be to synthesize a substance in water or an aqueous solution under high temperature and high pressure conditions. The hydrothermal reaction can increase the crystallinity by dispersing the PBA evenly in water and raising and raising the pressure. The hydrothermal reaction may be a temperature condition of 80 to 100 ℃, 85 to 100 ℃, 90 to 100 ℃, or 95 to 100 ℃. The time of the hydrothermal reaction may be 4 to 8 hours, 5 to 7 hours, or 5 and a half hours to 6 and a half hours.

The heat treatment step may be a heat treatment in an Ar atmosphere, the temperature conditions are 250 to 350 ℃, 260 to 350 ℃, 270 to 350 ℃, 280 to 350 ℃, 290 to 350 ℃, 250 to 340 ℃, 260 to 340 ℃ , 270 to 340 ° C, 280 to 340 ° C, 290 to 340 ° C, 250 to 330 ° C, 260 to 330 ° C, 270 to 330 ° C, 280 to 330 ° C, 290 to 330 ° C, 250 to 320 ° C, 260 to 320 ° C , 270 to 320 ° C, 280 to 320 ° C, 290 to 320 ° C, 250 to 310 ° C, 260 to 310 ° C, 270 to 310 ° C, 280 to 310 ° C, 290 to 310 ° C, and heat treatment time from 120 minutes to 240 minutes, 140 minutes to 240 minutes, 160 minutes to 240 minutes, 120 minutes to 220 minutes, 140 minutes to 220 minutes, 160 minutes to 220 minutes, 120 minutes to 200 minutes, 140 minutes to 200 minutes, or 160 minutes to 200 minutes It can be minutes. In the heat treatment step, the intermediate product obtained after the hydrothermal reaction may be highly crystallized.

The method for manufacturing a metal-organic framework-based water electrolytic catalyst may further include a second drying step of 50 to 90 ° C. for 10 to 14 hours after the heat treatment. The temperature conditions of the secondary drying may be 50 to 90 ° C, 60 to 90 ° C, 50 to 80 ° C, 60 to 90 ° C, preferably 65 to 75 ° C, and the drying time is 10 to 14 hours, It may be 11 to 14 hours, 10 to 13 hours, and preferably 11 to 13 hours.

The second drying step may be a vacuum condition, for example, it may be dried using a vacuum oven.

Hereinafter will be described in more detail through an embodiment of the present invention. However, these examples are for illustrative purposes only, and the present invention is not limited to the following examples.

Hereinafter, in the examples of the present invention, manufacturing examples, and experimental examples, the metal organic structure may be referred to as MOF. The metal organic structure (MOF) according to an embodiment of the present invention has a function of a water electrolytic catalyst itself. Therefore, in this specification, the metal organic structure-based water electrolytic catalyst may be referred to as a metal organic structure.

<Production Example: Prussian blue analog (PBA) synthesis>

Prussian Blue Analog (PBA) is used to synthesize metal-organic frameworks.

NC-PBA

The Prussian blue analog containing Ni, Co (hereinafter referred to as NC-PBA) may be represented by the formula of Ni ₃ [Co (CN) ₆ ] ₂ . The NC-PBA is used as a precursor for synthesizing a metal organic skeleton containing Ni and Co.

An example of the crystal structure of the NC-PBA is shown in Fig. 1A. The appearance of the NC-PBA is shown in A and B of FIG. 2.

NF-PBA

Prussian blue analogs containing Ni and Fe (hereinafter referred to as NF-PBA) may be represented by the formula Ni ₃ [Fe (CN) ₆ ] ₂ . The NF-PBA is used as a precursor for synthesizing a metal-organic framework containing Ni and Fe. An example of the crystal structure of the NF-PBA is shown in Fig. 1B. The appearance of the NF-PBA is shown in FIGS. 2C and D.

NCF-PBA

Prussian blue analogs containing Ni, Co, Fe (hereinafter referred to as NCF-PBA) may be represented by the formula of Ni ₃ [Fe _x Co _1-x (CN) ₆ ] ₂ . The NCF-PBA is used as a precursor for synthesizing a metal-organic framework containing Ni, Co, and Fe. An example of the crystal structure of the NCF-PBA is shown in FIG. 1C. The appearance of the NCF-PBA is shown in E and F of FIG. 2.

Preparation Example 1: NCF-PBA synthesis

1) First solution preparation: A first solution was prepared by dissolving 0.6 nmol of Nickel Nitrate and 0.9 mmol of sodium citrate in 20 ml of Mili-Q water.

2) Preparation of the second solution: 0.2 mmol of potassium hexacyano ferrate (III) (potassium hexacyanoferrate (III)) and 0.2 mmol of potassium hexacyano cobaltate (III) (potassium hexacyano cobaltate (III)) with Mili-Q water A second solution was prepared by dissolving in 20 ml.

3) Third solution preparation: A third solution was prepared by mixing the first solution and the second solution.

4) Coprecipitation: Stirred at 300 rpm at room temperature for 12 hours to coprecipitate to obtain NF-PBA.

Preparation Example 2: NC-PBA synthesis

The second solution was prepared by dissolving 0.4 mmol of potassium hexacyanocobaltate (III) (potassium hexacyano cobaltate (III)) in 20 ml of Mili-Q water. The remaining steps 1), 3) and 4) were obtained in the same manner to obtain NC-PBA.

Preparation Example 3: NF-PBA synthesis

The second solution was prepared by dissolving 0.4 mmol of potassium hexacyanoferrate (III) in 20 ml of Mili-Q water. The remaining steps 1), 3), and 4) obtained NCF-PBA in the same manner.

The Mili-Q water refers to ultra pure water in which almost no ions are present. The citrate ions are intended to prevent the formation of excessively large particles during the coprecipitation process. Stirring in step 4) was carried out for 12 hours, to obtain a small and uniform PBA precursor. The stirring time is not absolute, and may have an appropriate range depending on other conditions.

<Example: NCF-MOF production>

Using the Prussian blue analog (PBA) as a precursor, a metal-organic framework-based water electrolytic catalyst was prepared therefrom. The metal organic skeleton-based water electrolytic catalyst may be referred to as MOF. The metal-organic framework-based water electrolytic catalyst prepared from NC-PBA may be referred to as NC-MOF. The metal-organic framework-based water electrolytic catalyst prepared from NF-PBA may be referred to as NF-MOF. The metal-organic framework based water electrolytic catalyst prepared from NCF-PBA may be referred to as NCF-MOF.

Example: NCF-MOF production

100 mg of the NCF-PBA precursor and 200 mg of Na ₂ S were mixed in 100 mL of ethanol. Hydrothermal reaction was performed for the ion exchange reaction. The mixture was transferred to a Teflon Liner Stainless-Steel Autoclave for hydrothermal reaction. The autoclave was subjected to hydrothermal reaction at 100 ° C. for 6 hours to induce sufficient ion exchange.

The intermediate product obtained after the hydrothermal reaction was washed with a mixed solution of water and ethanol and dried. After drying, it was pulverized with Agate Motar to make an intermediate product powder.

The powder was heat-treated at 300 ° C. for 3 hours on Ar gas to prepare highly crystalline Highly Crystalline Frameworks (MOFs). The heat treatment may improve crystallinity.

The final NCF-MOF was prepared by pulverizing the high-crystalline MOF and drying the high-crystalline MOF in a vacuum oven at 70 ° C. for 12 hours. The drying is to remove moisture that may be introduced in the grinding process.

Comparative Example 1: Preparation of NC-MOF

The NC-PBA precursor was used, and the rest of the procedure was the same as that of NCF-MOF preparation to prepare NC-MOFs.

Comparative Example 2: NF-MOF production

The NF-PBA precursor was used, and the rest of the procedure was the same as that of NCF-MOF preparation to prepare NF-MOFs.

The final MOF was referred to as NC-MOF, NF-MOF, and NCF-MOF, respectively.

<Experimental Example 1: Average size, pore size, external form>

The average size, pore size and external morphology of the metal organic framework (MOF) based electrolytic catalyst were observed by SEM.

The prussian blue analog used as a precursor for the synthesis of the metal-organic framework-based water electrolytic catalyst has a shape close to a cube when the shape is confirmed by SEM. In the present specification, a shape close to a cube that can be observed with the SEM may be referred to as a nanocube shape. (See Fig. 2) It can be explained that the particles of each of the metal-organic frameworks maintain a nano-cube shape when the difference from the PBA is not significantly different. (See Figure 3)

It will be described with reference to SEM pictures of NC-MOF shown in A, B of FIG. 3. NC-MOF is in the form of nano cubes. The average size is 400 nm, and there are pores larger than NF-MOF and NCF-MOF on each side.

It will be described with reference to SEM pictures of NF-MOF shown in D, E of FIG. 3. NF-MOF has an average size of 160 nm. The exterior is similar to that of the PBA precursor. External voids were not observed well compared to NC-MOF and NCF-MOF.

It will be described with reference to SEM pictures of the NCF-MOF shown in G, H of FIG. 3. NCF-MOF has an average size of 180 nm, has a rough surface, and maintains a general nanocube shape. The rough surface of the NCF-MOF may suggest that there is an adequate void to the extent that the nanocube shape can be maintained outside the nanocube.

It can be seen that the shape of the MOF is highly dependent on the type of transition metal center present in the PBA.

The size of MOF was relatively largest in NC-MOF. This is thought to be due to the absence of Fe.

NC-MOF had the largest air gap in MOF. This takes place in the hydrothermal reaction step _{^{[Co (CN) - 6]}} 3 - ions and the speed of the S ^2- ion exchange reaction of the ions may be due to a relatively fast in the NC-MOF. Due to the relatively fast ion exchange reaction, relatively large pores may be formed inside the NC-MOF. If the pores are too large, the specific surface area of NC-MOF may decrease, the exposed catalytically active sites may decrease, and the structural durability of NC-MOF may be reduced.

NF-MOF and NCF-MOF retain the nano-cube shape than NC-MOF. NF-MOF and NCF-MOF have fewer morphological changes that can be observed compared to PBA compared to NC-MOF.

This [Fe (CN) _6] ^3- ion and S ^2- speed of the ion exchange reactions of ions thought to be due to lower than the speed of the ions and the ion-exchange reaction of S ^2- ions ^{_{- [Co (CN) - 6}} ] 3 do.

Therefore, the porosity of MOF and the external form of MOF can be controlled according to the content of Co and Fe. If the MOF does not contain Fe, the porosity may be too low, and if the MOF does not contain Co, the pores may be too large and the durability required as a metal-organic framework-based water electrolytic catalyst may be low.

<Experimental Example 2: Internal morphology analysis>

The internal morphological characteristics of MOF were analyzed by TEM.

Referring to C of FIG. 4, in NC-MOF, voids observed on each side of the nanocube were connected inside to form hollows.

Referring to F of FIG. 4, almost no pores were observed in the NF-MOF.

Referring to I of FIG. 4, a number of voids were observed in the NCF-MOF, and hollows were formed.

Therefore, it was found that porosity and hollow formation in the MOF can be controlled by adjusting the contents of Co and Fe.

The MOF with the appropriate size of the hollow can contact more reactants, enable smooth inflow and discharge of a larger amount of reactants, and can smoothly discharge hydrogen or oxygen gas generated in the water electrolysis reaction. If the hollow size of the MOF is appropriate, exposure of metal sites with catalytic activity that can contact the reactants can be increased and the electrochemical performance of the MOF can be optimized.

<Experimental Example 3: Crystal structure analysis>

X-ray diffraction analysis (XRD) (AXS D8 Advancer, Bruker) was performed to confirm the crystal structure of the PBA precursors and final MOFs. XRD was a condition of Cu Kα radiation (λ = 1.5405 Å), and was measured at a scan rate of 10 ° to 80 °, 0.02 ° min ⁻¹ in a 2θ range at 0.02 ° interval.

The crystal structure of MOF was observed through X-ray diffraction (XRD).

This will be described with reference to FIGS. 5 and 6. The XRD pattern of NC-MOF was consistent with that of NiS. Thus NC-MOF is almost all of it was found that it has the S ^2- ion-ion and ion-exchange reaction _{^{- [Co (CN) - 6}} ] 3. The XRD pattern of NF-MOF was consistent with typical characteristic peaks of NF-PBA. The XRD pattern of NCF-MOF was consistent with typical characteristic peaks of NCF-PBA.

Through the crystal structure analysis through the XRD, it was found that the change in the external form is the difference in the rate of the ion exchange reaction of S ^2- .

<Experimental Example 4: Analysis by EDS Line Scanning>

The morphology and final element distribution of MOF were analyzed using EDS line scan and mapping analysis using SEM (LEO FESEM 1530) and HRTEM (JEOL 2010F, JEOL Ltd.).

For the analysis of the components of the MOF, Energy-Dispersive X-ray Spectrometry (EDS) Line-Scans were performed by scanning TEM (STEM).

This will be described with reference to FIGS. 7 to 10.

7 and 8, NC-MOF confirmed the porous structure and the hollow structure to which the inside is connected.

7 and 9, NF-MOF has a higher density and no hollow structure than NC-MOF.

7 and 10, it was confirmed that the NCF-MOF has a porous structure and a hollow structure.

<Experimental Example 5: Structure analysis by SAED>

This will be described with reference to FIG. 11.

NC-MOF was analyzed by a face-centered cubic lattice structure (FCC Lattice Structure). NF-MOF was analyzed with a polycrystalline FCC structure. NCF-MOF was analyzed to correspond to a typical face-centered cubic structure (FCC) structure.

<Experimental Example 6: Elemental mapping analysis by DFEEM>

This will be described with reference to FIG. 12.

NC-MOF consists of Ni, N, S, C and residual Co. S was measured a lot outside and inside, and Co was hardly measured. It can be seen that the S ^2- ion can be seen that the ions, because of the fast ion exchange reaction rate, and also because the hollow of the NC-MOF the fast ion exchange reaction rate _{^{- [Co (CN) - 6}} ] 3.

NF-MOF contained more Ni and Fe than NC-MOF both inside and outside, and less S than NC-MOF.

It can be seen that the content of S of NF-MOF is smaller than that of NC-MOF due to less S ² ion exchange.

NCF-MOF is because the Co and Fe are uniformly distributed in the outside and inside of the nano-cube by containing _{^{Fe [Co (CN) - 6}} ] 3 - ions and the S ^2- ion exchange reaction of ions as compared with the NC-MOF It can be seen that only a part has happened.

<Experimental Example 7: Analysis of porosity of MOF>

As a result of analyzing the N ₂ adsorption and desorption isotherms of NF-MOF and NCF-MOF, it shows a hysteresis loop in the pressure range of 0.6 to 1.0, and through this, Mesopore is present in NF-MOF and NCF-MOF. Able to know. (See Figure 13)

The specific surface area was analyzed by the BET (Brunauer-Emmett-Teller) method. NF-MOF was 27.4 m ^{2 ·} g ^-1 , and NCF-MOF showed a larger 50.7 m ^{2 ·} g ^{- 1} than NF-MOF. (See Figure 14)

The pore size distribution and pore volume were analyzed by BJH (Barrett-Joyner-Halenda) method. The cumulative volume of voids NCF-MOF is 0.29cc ^· g ^- a 1, NF-MOF is 0.14cc ^· g ^{- 1} because the measurement was found that MOF-NCF more Mesopore is present than NF-MOF. (See FIGS. 11 and 14)

As a result of the specific surface area and pore analysis, it can be seen that NCF-MOF can significantly increase the activity of HER and OER by exposing more active sites that catalyze the water electrolysis reaction.

<Experimental Example 8: Composition of the MOF catalyst (Coposition of the MOF catalyst)>

XPS (K-Alpha XPS spectroscopy, Thermal Scientific) analysis was performed to confirm the oxidation state of the final MOFs and each binding group.

15 and 16, the peaks of Ni 2p (orbital) and S 2p were observed in the spectrum of NC-MOF, and thus binding energy corresponding to NiS and Ni ₃ S ₂ was observed. Since Ni ₃ S ₂ was not observed in the XRD pattern described later in the NC-MOF, it is considered to be a negligible amount.

15 and 16, based on the analysis of the Ni 2p and Fe 2p signals of the spectrum of the NF-MOF, mainly Ni ^{2 +} and Fe are Fe ^{2 +} about 27% and Fe ^{3 +} about 73% based on the total Fe atom. It was confirmed to include. The chemical composition of NF-MOF can be expressed as Ni _x ^II [Fey ^II Fe _z ^III (CN) ₆ ] ² .

15 and 17, in the spectrum of NCF-MOF, Ni ^{2 +} and Fe include Fe ^{2 +} about 43% and Fe ^{3 +} about 57% based on the total Fe atom, and Co is based on the total Co atom. It was confirmed to include Co ^{2 +} about 17% and Co ^{3 +} about 83%. Referring to the Ni 2p peak and the S 2p peak of the NCF-MOF, a small amount of NiS is formed. NiS was found to be about 5.4% based on total Ni atoms. This is consistent with the results observed in SEM, TEM and XRD. Since there is no characteristic NiS peak in the XRD pattern analysis of NCF-MOF, it can be determined that the NiS of NCF-MOF is amorphous.

<Experimental Example 9: Calculation of the outermost electron number of MOF>

Based on the oxidation state determined through XPS analysis, the average number of outermost electrons of Fe and Co included in the MOF was calculated.

[Fe of NF-MOF]

2+: 6 × 0.733 = 4.398

3+: 5 × 0.267 = 1.335

The total number of outermost electrons of Fe in NF-MOF is 4.398 + 1.335 = 5.73.

[Fe _{0 .5} the NCF-MOF]

2+: 6 × 0.427 = 2.562

3+: 5 × 0.573 = 2.865

[NCF-MOF Co _{0 .45} ]

2+: 7 × 0.173 = 1.211

3+: 6 × 0.827 = 4.962

The total number of outermost electrons of Fe and Co of NCF-MOF is 2.71 + 2.78 = 5.49.

In the OER reaction, the larger the number of outermost electrons of the MOF transition electrons, the greater the adsorption energy of oxygen intermediates such as * OH and * OOH, and the higher the overvoltage for OER. Since the outermost electron number of MCF-MOF is lower than the outermost electron number of NF-MOF, it can be seen that the overvoltage for OER of NCF-MOF is lower.

<Experimental Example 10: nitrogen doping (N-Doping) confirmation>

Whether NC-MOF, NF-MOF, and NCF-MOF was nitrogen doped was confirmed by analyzing the XPS spectrum.

15, N 1s peak is not detected in the XPS spectrum of NC-MOF. This may be because ion exchange of [Co (CN) ₆ ] ^3- ions and S ^2- ions occurs actively, and may be because Ni ₂ [Fe (CN) ₆ ] ³ does not exist.

15, N 1s peak is detected in the XPS spectrum of NF-MOF. Based on the number of NF-MOF total nitrogen atoms, CN of [Fe (CN) ₆ ] ^3- is 98.7% and nitrogen oxide 1.9%. This is because [Fe (CN) ₆ ] ^3- rarely undergoes ion exchange reaction with S ² .

Referring to FIG. 15, N 1s peaks are detected in the XPS spectrum of the NCF-MOF. Based on the number of NCF-MOF total nitrogen atoms, [Fe (CN) ₆ ] ^3- and [Co (CN) ₆ ] ^3- CN is 94.7%, nitric oxide 0.9%, and pyridine nitrogen 4.24%. The pyridine nitrogen may be generated from free CN ^- present in the solution by an ion exchange reaction of [Co (CN) ₆ ] ^3- ions and S ^2- ions of NCF-MOF. Free CN ^- in solution is oxidized to CNO ^- by Ni ₃ [Fe (CN) ₆ ] ² and can be further decomposed to form NH ₃ . NH ₃ may play a role in doping nitrogen. The nitrogen oxide may be in a form in which oxygen is bonded to pyridine nitrogen.

In the XPS analysis, 9.85% of [Co (CN) ₆ ] ^3- ions were ion-exchanged with S ^2- ions in NCF-MOF, and 86% of CN ^- present in the solution was ion-doped with nitrogen. This result is also consistent with the EDS line scan analysis of the NCF-MOF in FIG. 10.

The doped nitrogen of NCF-MOF can improve HER activity by synergy with transition metals such as Ni, Co and Fe.

<Experimental Example 11: Measurement of HER and OER activity of MOF>

The electrochemical properties of MOF as an electrocatalyst were analyzed using a typical half-cell system with three electrodes. The three electrodes are a working electrode, a glass carbon electrode (GCE), a reference electrode (saturated calomel electrode, SCE), and a counter electrode (Counter electrode), a platinum wire (platinum wire) was used.

The electrocatalytic activity for OER and HER was evaluated using the Rotating Disk Electrode (hereinafter referred to as RDE) method using the MOF water electrolytic catalyst loaded NFE or catalyst coated GCE.

CHI Electrochemical Station (Model 760D) was used for all electrochemical evaluations. OER measurements were made from 0.00 to 0.85V for SCE at a rotational speed of 1600 rpm. The HER measurement was measured at -0.9 to -1.5 V compared to SCE at a rotational speed of 1600 rpm. The scan speed was performed at 10 mV · s ^-1 . Pure nitrogen gas was purged for 30 minutes prior to OER and HER measurements, and all measurements were performed in 0.1 m KOH solution.

All measured potentials were adjusted to the Reversible Hydrogen Electrode (RHE) scale by Equation 1 below.

[Equation 1]

E _RHE = E _SCE + 0.241 + 0.059 pH

All measured polarization curves were iR-compensated prior to electrochemical testing. GCE (Glassy Carbon Electrode) is 5mm in diameter. The GCE was polished uniformly and smoothly using alumina suspension (size: 0.05 μm) on a smooth polishing cloth. Then, the surface of the polished GCE was sonicated in Mili-Q water for several seconds to remove the remaining alumina particles.

The catalyst ink is a Nafion solution (IPA 10mL, Nafion 5 wt%, 80μl (Ion Power), which is pretreated with 2mg of a metal-organic framework-based water electrolytic catalyst (for example, NCF-MOF) and Vulcan Carbon (XC-72, VC). ) Dissolved in 1 ml and sonicated for 60 minutes to prepare a uniform suspension.

20 μl of the catalyst ink was dropped on the surface of the GCE, and dried at room temperature to load the catalyst by 0.2 mg · cm ⁻² . The 0.2mg / cm ² is a value obtained by dividing the amount of the catalyst in the solution by the area of the electrode, so that the content of Vulcan carbon is excluded. On the other hand, a noble metal catalyst composed of platinum (Pt / C 29.9 wt% Pt) and iridium (Ir / C, 20 wt% Ir) is used as benchmark catalysts for comparing the electrocatalytic performance and actual efficiency of MOF catalysts. Became.

Referring to the HER of FIG. 18, based on the HER polarization curve, NC-MOF represents -0.39V (meaning a distance away from 0V) under a current density condition of -10mA · cm ² , and NF-MOF is − 0.34V (meaning the distance away from 0V). Under the same conditions, NCF-MOF exhibited an overvoltage of -0.27V (meaning a distance from 0V) lower than that of NC-MOF and NF-MOF. NCF-MOF may exhibit low overvoltage by reducing the HER energy barrier by doped nitrogen of NCF-MOF. Doped nitrogen can reduce ΔG (H *) by enhancing adsorption to H * at the catalytically active site. In addition, NCF-MOF may exhibit a low overvoltage because of its high specific surface area and high porosity, so that the catalytically active site can contact more water.

NCF-MOF exhibits lower overvoltage for OER compared to Ir / C catalyst, which is a noble metal catalyst. Referring to the OER of FIG. 18, NCF-MOF exhibited an overvoltage of 0.32 V under a current density condition of 10 mA · cm ⁻² . (0.32V means 0.32V is higher based on 1.23V. It means that the potential for water decomposition is 1.23V based on RHE, so it means 1.55V based on RHE.) NF-MOF is 30mV higher than NCF-MOF It showed overvoltage. NC-MOF showed an overvoltage of 60 mV higher than NCF-MOF. Ir / C showed an overvoltage of 20 mV higher than NCF-MOF. The low overvoltage for the OER of the NCF-MOF may be attributed to the low number of outermost electrons of the transition metals Co and Fe included in the NCF-MOF.

<Experimental Example 12: Nickel Foam Electrode (NFE) test>

NFE (Nickel Foam Electrode) loaded with any MOF catalyst of NC-MOF, NF-MOF, and NCF-MOF was used as a working electrode. And it was tested using the three electrode configuration described above.

The NFE carrying MOF was prepared as follows. Cut NFE pieces to 1 cm to 2 cm in size. In addition, the oxidized nickel was removed by washing in a 2.0 m HCl aqueous solution for 1 hour while sonication. Again, ultrasonic cleaning was performed in acetone for 1 hour to remove organic impurities on the surface of NFEs.

MOF ink is mixed with 0.5 mg of MOF and 0.5 mg of VC (Vulcan Carbon) in 1.0 mL of pre-treated Nafion solution (10 mL of isopropyl alcohol (IPA) and 80 μl solution of Nafion 5 wt%, Ion Power), and ultrasonically treated for 60 minutes. One solution was obtained. Then, the mixture was drop-cast on dried NFE (active area 1 cm ² ) to support 0.5 mg · cm ^{−2 of} the catalyst.

The Nafion is an electrolyte for a proton exchange membrane and is well known to a person skilled in the art as a product of DuPont. The pre-treatment of the Nafion is well known to those skilled in the art using Nafion, so a detailed description thereof will be omitted.

Cycling was performed at 50 mV · s ⁻¹ scan rate in N ₂ saturated electrolyte until the CV curve showed a stable current representing a stable solid / liquid interface.

Referring to FIG. 19, under the HER condition, the NFE carrying NC-MOF at a current density of -30mA · cm ^-2 exhibited an overvoltage of 0.30V in the HER. Under the same conditions, NFE carrying NF-MOF showed an overvoltage of 0.25V. The NFE carrying NCF-MOF showed a lower overvoltage of 0.16V than the NFE carrying the NC-MOF or NF-MOF.

Referring to FIG. 19, in the Tafel analysis from the HER curve, the NFE carrying NC-MOF was 168 mV · dec ^{- 1} and the NFE carrying NF-MOF was 157 mV · dec ^{- 1} . The NFE carrying NCF-MOF was 114 mV · dec ^-1 . The Tafel slope in the HER of the NCF-MOF was lower compared to the NFE carrying the NC-MOF and the NFE carrying the NF-MOF. The lower the slope of the Tafel slope, the lower the voltage required to reach hydrogen production.

Referring to FIG. 20, under the OER condition, at a current density of 30 mA · cm ⁻² , NCF-MOF exhibited an overvoltage of 0.48 V. The overvoltage in the OER of the NCF-MOF was 160 mV lower than that of Ir / C, and 110 mV lower than that of NF-MOF.

Referring to FIG. 20, in the OER Tafel analysis, NFE carrying NC-MOF is 99 mV · dec ^-1 , NFE carrying NF-MOF is 119 mV · dec ^-1 , and Ir / C is 73 mV · dec ^-1 . The NFE loaded with NCF-MOF was 49 mV · dec ^-1 , showing a lower slope than Ir / C.

Referring to FIG. 21, NFE without MOF catalyst as a control showed a much higher overvoltage in both HER and OER than NFE with MOR catalyst.

<Experimental Example 13: Electrochemical safety test>

22 and 23, two electrochemical durability tests were performed.

The first was to check the durability by passing a fixed current through Chronopotentiometry, and measuring the potential value according to it to measure how much the value changed compared to the initial value. In this way, durability was tested in both the HER and OER potential ranges.

The second was the durability test related to OER. Since the OER has a higher potential than the HER condition and is an oxidative condition, it is a more severe condition for the catalyst, so the durability test of the OER was conducted separately. Durability was tested by comparing the polarity curves before and after the cycle by giving repeated cycles within the OER potential range.

Chronopotentiometry (CP) was performed to confirm the electrochemical stability of the MOF catalyst.

First, GCE coated with MOF catalyst was activated by cyclic voltammetry (about 40 cycles) in nitrogen saturated electrolyte.

The HER of the MOF catalyst was subjected to CP evaluation at a current density of -20mA · cm ^-2 . The OER of the MOF catalyst was subjected to CP evaluation at a current density of 20 mA · cm ^-2 .

For the electrochemical performance measurement, the overvoltage of the OER of the MOF catalyst coated GCE was measured at 10 mA · cm ^-2 , and the HER overvoltage of the MOF catalyst coated GCE was measured at -10 mA · cm ^-2 .

The OER overvoltage of the NFE carrying the MOF catalyst was measured at 30 mA · cm ^-2 , and the HER overvoltage of the NFE carrying the MOF catalyst was measured at -30 mA · cm ^-2 . For easier comparison of OER and HER overvoltages, the standard potential for OER was 1.23V vs RHE, and the standard potential for HER was 0.00V vs RHE.

Referring to FIG. 22, in the HER CP evaluation conditions, the NCF-MOF electrode exhibited an initial HER potential of -0.34V based on RHE, and a voltage loss of 0.05V after 20000s. This indicates that NCF-MOF retains the potential by 86% after 20000s in HER.

22 and 23, in the OER CP evaluation conditions, the NCF-MOF electrode exhibited an initial OER potential of 1.53 V compared to RHE, and an OER potential increase of 0.01 V after 20000 s. This indicates that the NCF-MOF retains the potential by 97% after 20000s in the OER, and has excellent electrochemical durability.

The OER CP evaluation of NCF-MOF was compared with Ir / C.

Referring to FIG. 22, in the OER, the NCF-MOF exhibited an initial OER potential of 1.54 V compared to the RHE. Under the same conditions, Ir / C had an initial OER potential of 1.57V. NCF-MOF showed an initial OER potential lower than Ir / C for OER.

After 1000 CV cycles at a high oxidation potential window of 1.2 to 1.75 V versus RHE, NCF-MOF exhibited a potential loss of 1 mV and a potential retention of 99%. Under the same conditions, Ir / C exhibited a potential loss of 116 mV and a potential retention rate of 66%.

<Experiment 14: confirm the structural safety of NCF-MOF>

After the electrochemical safety test, structural safety was confirmed through TEM characterization of NCF-MOF.

Referring to FIG. 24, after the OER and HER electrochemical safety tests in the alkaline electrolyte solution, it was confirmed that the initial form of the whole nanocube of NCF-MOF is maintained. The maintenance of the initial form may be useful as an electrochemical catalyst because the crystal structure is maintained even after side reactions that may occur with HER, OER, or for a long time. The corners of the nanocube are slightly roughened. This may be due to the formation of a metal hydroxide at an undercoordinated metal site. In more detail, it may be because a metal that does not coordinate with surrounding ligands forms a metal hydroxide.

The electrochemical safety of NCF-MOF in HER and OER may be due to the rigid porous structure capable of maintaining the porous skeleton during the generation of hydrogen gas and oxygen gas. In addition, an overvoltage lower than the Ir / C of NCF-MOF in OER reduces carbon corrosion and reduces the oxidation of active metals. The carbon corrosion is a phenomenon in which carbon is released from the high potential region of the OER, and may mean, for example, a phenomenon of changing to CO or CO ₂ . Due to the low overvoltage in the OER, carbon corrosion of the NCF-MOF can be reduced because the carbon in the lower region is exposed to the potential.

Claims (20)

1. Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} is represented by the formula of X is 0.1 to 0.9 is prepared from a Prussian Blue Analog (PBA),

   As the transition metal, Ni of 2 oxidized water, Co of 2 oxidized water, Co of 3 oxidized water, Fe of 2 oxidized water, and Fe of 3 oxidized water,

   [Co (CN) ₆ ] ^3- coordination compound in which complex ion and Ni ²⁺ are combined,

   [Fe (CN) ₆ ] ^3- Coordination compound in which complex ion and Ni ²⁺ are combined, and

   The [Co (CN) ₆ ] metal organic skeleton based water electrolytic catalyst containing NiS generated by the ion exchange reaction of ^3- complexion and S2 ^- ion.
2. According to claim 1,

   Based on XPS analysis, the metal-organic framework-based water electrolysis catalyst has a higher content of Fe in the oxidation number 3 than the content of Fe in the oxidation number 2.
3. According to claim 1,

   The content of Fe in the oxidation number 2 is 40 to 46% by weight based on the total amount of Fe metal-organic framework based water electrolytic catalyst.
4. According to claim 1,

   Metal-organic framework-based water electrolytic catalyst further comprising doped nitrogen.
5. The method of claim 4,

   At least some of the doped nitrogen is a pyridine nitrogen metal organic skeleton based water electrolytic catalyst.
6. The method of claim 4,

   At least some of the doped nitrogen is a metal-organic framework-based water electrolytic catalyst that is nitrogen derived from CN ^- generated by the ion exchange reaction.
7. The metal-organic framework-based water electrolytic catalyst according to claim 1, wherein the specific surface area (BET) is 30 to 60 m ^{2 ·} g ^-1 .
8. The metal-organic framework-based water electrolytic catalyst according to claim 1 comprising a void.
9. The method of claim 8,

   At least some of the pores are mesoporous metal-organic framework-based water electrolytic catalyst.
10. The method of claim 8,

    At least some of the pores are [Co (CN) ₆ ] metal organic skeleton-based water electrolytic catalysts, which are pores generated by ion exchange reactions of ^3- and S ^2- ions.
11. According to claim 1,

    The total pore volume is 0.15 to 0.35 cc · g ^-1 of the metal-organic framework based water electrolytic catalyst.
12. According to claim 1,

    The metal organic skeleton based water electrolytic catalyst having a sum of the outermost electron number of Fe of 2 oxidized water, Fe of 3 oxidized water, Co of 2 oxidized water, and Co of 3 oxidized water.
13. According to claim 1,

    Ni combined with S of the NiS is a metal-organic framework-based water electrolytic catalyst containing 1 to 10% based on the total Ni.
14. According to claim 1,

    The metal organic skeleton-based water electrolytic catalyst is (200) plane, (220) plane, (400) plane, (420) plane, (422) plane, (440) plane, (600) plane, (XRD) 620) Metal-organic framework-based water electrolytic catalyst with a peak observed on the surface.
15. A catalyst ink comprising the metal organic skeleton-based water electrolytic catalyst of claim 1.
16. Ni ₃ [Fe _X Co _1-X (CN) ₆ ] _{2 -} is represented by the formula of X is 0.1 to 0.9 to prepare a Prussian blue analog (PBA);

    Ion exchange reaction of [Co (CN) ₆ ] ^3- complex ions and S ^2- ions of the Prussian blue analog;

    Washing and drying the intermediate product subjected to the ion exchange reaction;

    Crushing the dried intermediate product; And

    A method of manufacturing a metal-organic framework-based water electrolytic catalyst comprising the step of heat-treating the crushed intermediate product.
17. The method of claim 16,

    Preparing the Prussian blue analog (PBA),

    Dissolving nickel nitrate and sodium citrate in water to prepare a first solution;

    Preparing a second solution by dissolving potassium hexacyanoferate (III) in the X ratio and potassium hexacyanocobaltate (III) in the 1-X ratio in water;

    Preparing a third solution by mixing the first solution and the second solution; And

    The method comprising the step of obtaining a Prussian blue analog (PBA) by coprecipitating the third solution, a method for producing a metal-organic framework-based water electrolytic catalyst.
18. The method of claim 16,

    The ion exchange reaction

    Mixing the Prussian blue analog with Na ₂ S, and

    Method for producing a metal-organic framework-based water electrolytic catalyst comprising the step of hydrothermal reaction of the mixture.
19. The method of claim 16,

    The heat treatment step is a metal-organic framework-based water electrolytic catalyst manufacturing method for heat treatment at 250 to 350 ℃ for 120 minutes to 240 minutes in an Ar atmosphere.
20. The method of claim 16,

    A method of manufacturing a metal-organic framework-based water electrolytic catalyst further comprising a second drying step of 50 to 90 ° C. for 10 to 14 hours after the heat treatment.

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